BT’s ISDN Switch Off – Are You Ready?

If you’ve been using ISDN or PSTN networks for your telephone and data needs, it’s probably a good time to begin packing for migration to the more advanced and efficient IP technology. This follows BT’s announcement in 2015 which stated that by 2020, the communication giant would begin the process of phasing out the legacy networks. The first phase, which involves ceasing installations and the supply of the old network tools begins in 2020. The move is meant to usher in the latest technology which uses internet protocol (IP) to transfer voice and other data services. By 2025, all entities which depend on ISDN or PSTN networks will no longer be able to use them.

Luckily, the switch off and migration to IP technology comes at an appropriate time considering that high internet speeds are now readily available and also affordable. While the traditional phone lines (ISDN and PSTN networks) support simultaneous delivery of voice and data services, they can be quite costly to use. Besides, they require bulky hardware and a range of other drawbacks that IP technology already solves. With this in mind, there’s every reason to consider VoIP as the future and the reason there’s no better time to begin preparation to switch than now.

If at this point you’re still wondering what this talk on ISDN and PSTN switch off is all about, we’ll take you a little back and discuss what these legacy networks entail.

What is ISDN?

Abbreviating for Integrated Service Digital Network, ISDN is a traditional telephone network system that’s circuit-switched to allow simultaneous relay of both voice and data services over digital lines. The system was established in 1986 as part of phasing out the analogue landline technology which was mainstream back then. The launch of ISDN excited many businesses since the network supported both analogue phone lines and video-conferencing systems that existed at the time. The company also offered a 128kbps internet which was extremely fast during those days.

With ISDN offering a combination of faster speeds and better connection, data transfer was significantly improved. Furthermore, the technology provided an overall enhanced experience in telephone communication as distortions and downtimes became minimal compared to the hitherto used systems.

History of ISDN

In the 1960s, the analogue phone lines began showing signs of unreliability especially over long distances, and this necessitated the search for an alternative and a faster way to relay voice. By the 1980s, a lot of research had been conducted and new innovative methods were being floated for implementation.

In 1988, the International Telecommunications Union was tasked with the responsibility of standardising voice and data communication amongst telecommunication companies. It is during this time that the body began recommending ISDN to be the standard network. Even so, the recommendations took a while before all the service providers implemented ISDN on their infrastructure. The use of varying operating systems was to blame for the slowed implementation.

In 1990, the standardisation efforts were now bearing fruits as the National ISDN 1 was fashioned. The realisation that the quality of voice and data carriage would improve significantly was a powerful motivation for the service providers to jump on board for the idea. But the standard had its fair share of drawbacks as implementation meant deeper digging into the pocket for the upgraded equipment. It was at that point that manufacturers like the U.S Robotics and Motorola set out to make the transition easier for everyone by developing the standards for ISDN equipment.

When the ISDN standardisation process was successfully executed, other major companies including BT adopted the technology. This not only lowered telecommunication pricing but also facilitated the internet access at a higher bandwidth. By the turn of the new millennium, ISDN had completely overhauled PSTN and had become the main voice and data routing network.

Few years into the millennium, the internet had grown tremendously thereby calling for an increment of the bandwidth. This saw the rise of more internet access connections including Cable Modems, DSL, and WAN which were capable of handling more bandwidth (over 128kbps). 

How ISDN Works

ISDN has been an excellent technology for internet access when options like WAN, DSL, and Cable Modems are unavailable. The concept behind how it works is easy to understand as you’ll see in our brief explanation below. However, before then, let’s first familiarise with PSTN which is the network system that ISDN heavily relied on especially when it first started out.

What is PSTN?

If you’re familiar with the landline telephone system, then you definitely have seen PSTN in use. The abbreviation PSTN stands for Public Switched Telephone Network with its origin dating back to the times when the invention of the first telephone occurred. The system utilised analogue technology to make voice communication possible. When ISDN was launched, it became a complementary system that allowed users to digitally transfer voice and video data.

PSTN uses copper wires to carry the analogue voice data and is controlled by circuit-switched phone lines. Although ISDN relays data digitally, it relies on PSTN infrastructure to facilitate voice communication.

ISDN Setup

For it to work, ISDN has to be connected to the traditional Plain Old Telephone Service (POTS) line that has access to both analogue and digital phone numbers. Transfer of voice and data occurs through a serial port where the phone line plug is inserted.

To make it work, you have to:

  • Program the modem and load the driver disk
  • Point the modem to both phone numbers
  • Configure the connection speeds for each line
  • Set the modem to dial the ISP to initiate the internet connection
  • Optionally activate BONDING, a technology that increases internet connection speeds by dialling both phone numbers concurrently.

ISDN Types

ISDN network is available in two options namely the Primary Rate Interface (PRI) and Basic Rate Interface (BRI). The main distinction between them lies in their reliability and the level of service that each offers. BRI is cheaper and is suitable for those looking to meet the basic needs of the network. On the other hand, PRI gives faster speeds, a better connection, and is increasingly reliable over busier lines.

Both types utilise B and D channels to transmit data and other forms of communication. However, they vary in the number of transmission channels; BRI uses one D channel and 2 B channels to achieve its maximum speed of 128kbps. PRI can accomplish speeds of up to 2.94mbps considering its location on the system and the number of channels it uses. In most countries, PRI is configured to use 23 B channels and only one D channel although the latter can be doubled for backup purposes. The flexibility that PRI ISDN offers makes it preferable by most businesses and is the standard interface for the BT network currently.

Both ISDN types support BONDING and Multilink PPP which are commonly used by other internet protocol connections. BONDING allows you to utilise several ISDN B channels in a technique known as load balancing, channel aggregation, or multi-linking. As for Multilink PPP, data is split, recombined, and sequenced across several links for quicker transmission.

Benefits of ISDN

Even with the ongoing plans to switch off ISDN, we have to admit that the network technology has boasted its fair share of benefits, the same that boosted its popularity in the first place. Besides being utilised in areas where broadband internet was unavailable, it offered quite decent speeds at a time when the internet was still in its infancy.

Here is a summary of those ISDN benefits:

  • Utilises existing telephone lines to broadcast digital signals
  • ISDN allows a higher data rate to be transmitted with PRI
  • Makes concurrent operations possible via a single copper wire by employing multiple digital channels
  • Takes relatively less time to begin transmission of voice and other data services compared to other modems.
  • Allows connection and usage of multiple devices over a single line. You can connect your fax machines, credit card readers, and other multiplex devices to the system.

Why are ISDN and PSTN are Being Phased Out by BT?

The development of the newer broadband internet access connection and its successful deployment has necessitated the exit of the legacy system. In recent years, more and more users have been preferring VoIP to ISDN with most now using the latter as a backup phone system when the mainline fails.

In addition, both networks are old and archaic with very high maintenance costs. Remember, most of the infrastructure used to support both technologies was borrowed from the same designs of the 1800s when the first telephone systems were created. And while significant improvements have been made over the years, the old infrastructure that’s still being used is turning out to be too costly to maintain while better and cheaper alternatives are now available.

Another obvious shortcoming of ISDN is the low connection speeds. As we mentioned earlier, this network only supports speeds of around 2.94mbps – a significantly lower rate than those of modern broadband. With these speeds, it’s practically impossible to use the technology to access the internet fast enough. Moreover, close to half of the UK business population no longer uses ISDN with most now preferring IP systems to access the web.

BT envisions to put in place a technology that meets the business demands of the 21st century with the main focus being on the future. Pumping more investment on an obsolete connectivity system no longer looks viable particularly with the target market now beginning to notice the better benefits of IP services.

In a nutshell, the availability of the faster internet speeds, cheaper infrastructure, and the changing preferences and tastes of the customer base are some of the main reasons BT is pushing for the migration to VoIP technology.

ISDN usage statistics in the UK

A survey carried out in early 2017 indicated that over 2 million entities in the UK relied heavily on the ISDN network. What’s shocking though is that over a quarter of that number had no idea that the switch-off would take place. Of course, 2020 is only a few months away and the upcoming changes we just explained earlier will be a big blow to many if they don’t make the necessary adjustments by then. So, what’s the way forward for these businesses?

Alternative solutions to use in place of ISDN

With the era of ISDN quickly approaching its tail end, many business heads are actively searching for alternate telephone systems to migrate to. VoIP is certainly the natural replacement but the sea of acronyms and technologies associated with it is already perplexing many.

In this section therefore, we’ll walk you through the two commonly used VoIP tech variants namely SIP Trunking and Hosted PBX.

But first, what is VoIP technology?

An abbreviation for Voice Over Internet Protocol, VoIP is a broad term that denotes the use of the internet as opposed to traditional phone lines such as ISDN to make phone calls. Over the years since its inception, numerous other terms have been coined to refer to this tech including voice over broadband, IP telephony, internet telephony, IP communications, and broadband phone service among others. Essentially, all these terms point to the technique of voice transmission to another endpoint or telephone.

In its most basic form, the technology only needs a VoIP software and a VoIP-enabled phone. The two work harmoniously to enable you make and receive calls over the internet. The setup is particularly handy when you want to get rid of the clutter associated with hardline phones.

With VoIP, you don’t have to rely on mobile phones which necessitate altering your official phone number(s). Since this technology only transmits voice data, most providers offer great deals including unlimited minutes. With this, the overall running cost drops significantly especially if you compare VoIP with ISDN.

VoIP has resulted in mushrooming of protocol technologies with prime examples being the Skype Protocol and SIP technology which is an open standard. In both, usage is facilitated by an IP enabled hardware and an application in the form of software. Great examples of VoIP applications include Google Talk and Skype which run on modern devices with IP addresses such as computers, smartphones, and tablets.

Understanding Protocol in VoIP

Often, the term protocol is mentioned whenever we refer to voice communications over the internet. A protocol consists of digital rules that govern how message exchange occurs between digital devices such as computers and phones. Its purpose is to enable developers to have a well-defined format for successful message exchange.

Protocols are agreed upon and a standard developed to facilitate synchronisation of communication. Simply put, they allow different vendors to create endpoints (digital devices) that are capable of communicating with one another.

Why choose VoIP for your business?

As BT’s ISDN switch-off due date fast approaches, here are a few reasons to choose VoIP.

  • VoIP offers greater flexibility in a wide variety of environments. You only need a reliable source of fast internet and your options grow significantly..
  • Improves the quality of a call. VoIP allows for high definition audio which is absent in ISDN connections.
  • Cost-effective with riddance of line rental and availability of unlimited minutes by service providers.
  • The system is highly resilient and helps to facilitate gains on business agility and its productivity.
  • Enjoy greater control and accountability based on the analytics capacity that this system possesses.

What is SIP Trunking?

In the business world, SIP Trunking is increasingly becoming a telephony technology of choice for many particularly for the benefits it offers over ISDN. Besides its increased reliability, this technology is flexible and offer numerous cost-saving opportunities. Of course, there’s more than meets the eye which is why in this section we analyse SIP Trunking in detail.

SIP Trunking Defined

SIP Trunking is simply a telephony technology that allows you to send or receive multimedia over the internet. Put otherwise, it’s an application layer protocol that establishes and manages multimedia sessions over an Internet Protocol network.

Unlike VoIP which handles just voice calls, this technology converts voice, video and other data into packets ready to be transmitted over a network. The abbreviation SIP stands for Session Initiation Protocol while the term Trunking referring to the technique of carrying data packets.

A SIP Trunk is the overhauled line that transmits data signals via electrical currents to other endpoints. With it, a physical connection is not quite necessary as data is transferred to other IP systems electronically.

While it’s a variant of VoIP, SIP trunking doesn’t require an active computer or dedicated and separate VoIP gadgets for the calls to go through. Only a modem is needed to facilitate an internet connection. With ISDN ageing, SIP connectivity appears to be the standard for the modern phone systems in managing voice communications.

How is SIP trunking different from ISDN?

While both systems use the Public Switched Telephone Network (PSTN), SIP trunks relay voice and other data through the internet as opposed to the outdated phone lines. With this comes improvements in terms of data quality, transfer capacity, and the overall cost of infrastructure.

ISDN, on the other hand, uses physical connections to transmit voice and other data signals. You’ll need a physical circuit as well as other external manifolds to function correctly. In SIP Trunking, only a device capable of working with a modem is needed.

SIP Trunks are available both as primary and secondary telephone solutions. They take a shorter time to set up and generally have lower call failure rates. ISDN in most cases requires the input of the Internet Service Provider (ISP) to work.

The cost of calling is significantly lower when using SIP Trunks while the high infrastructural costs of ISDN escalates the calling rates. BT’s estimates the difference in cost between the two phone systems to be approximately 30-50%.

Benefits of SIP Trunking

Whether you’re using SIP Trunking in a business environment or a home-based office, there are many benefits linked to the technology:

Enhanced security

With SIP Trunks, you can easily include a dedicated network firewall that monitors inbound and outbound connections. This increases protection from fraud as you’re able to control calls received by your devices or sent from it.


As earlier mentioned, BT estimates that SIP Trunking will be 30-50% cheaper compared to ISDN. Another benefit comes in the form of free UK landline calls when you use a SIP phone system. This will help to significantly cut operational costs for businesses. Besides, the lower maintenance costs as a result of fewer setup hardware translate to lower charges to the end consumer.

Increased flexibility

SIP Trunking offers more control over phone numbers just like hosted VoIP. It allows businesses to run operations outside their regional borders easily with virtual numbers that accept local codes. With this, the call charges from your far-off customers will bear a local rate as opposed to an international one. The benefit is in allowing businesses to establish a great rapport in foreign markets by facilitating cheaper, quicker, and easier communication with customers. The need to set up a local office in those locations will therefore no longer be a matter of urgency.


Indeed, scalability is a major gain with SIP Trunking as businesses can tailor costs in accordance with their financial muscle. The technology allows you to add extra trunks or reduce them in relation to the volume of voice calls. This kind of scalability is not easy to achieve with ISDN considering the high cost of purchasing and installing extra fixed lines.

Increased reliability

SIP Trunking eliminates the need to depend on a single phone line which means you can enjoy the confidence that comes with knowing you’re covered even if some hitch occurs on the system you’re using currently. For example, if your main broadband internet develops some problems, you can quickly route calls to other internet sources and continue to operate normally. Put simply, this telephone system is highly reliability as long as you have multiple sources of internet connection (which is not difficult to find nowadays).

Shortcomings of SIP Trunks

  • Regular analysis of the bandwidth is needed to ensure that the quality of service in multimedia transmissions remains reliable.
  • The need for acquiring an entire set of new equipment when migrating to SIP technology increases the cost of investment for businesses running on a fixed budget.
  • The technology is still quite new which means it might require a little getting used to and other expenses like training for your staff on how to use it.

Hosted PBX (Cloud Telephony)

Hosted Private Branch Exchange (PBX) is a form of VoIP technology that supports telephone services without necessarily requiring you to own all the equipment it uses. The system simply routes calls to extensions which are supplied by a VoIP service provider who then forwards them to your target destination (endpoints).

This technology allows for uninterrupted communication between the office telephone network and the wider public telephone network. With it, you can work from anywhere in the world and still be covered by the office telephone system.

With hosted PBX, users retain functionalities like call transfer, automated attendant, voicemail services, automatic call distribution, anonymous call rejection, and call-waiting to name a few.

Moreover, add-on services including teleconferencing, mobility features, call recording, and caller ID can be implemented on the system without you having to upgrade your PBX handset. Put simply, this telephone system allows businesses to operate fluidly in various locations while working under a single umbrella. It’s the best upgrade for firms aiming to cut down on their telephony resource requirements while also enhancing productivity.

With BT’s impending switch-off of the largely popular ISDN services, hosted PBX is now on course to take over the business telephony bragging rights, and for a good reason. Depending on your service provider, this tech comes with several key benefits including:

  • Nearly zero maintenance costs especially when compared to ISDN
  • Very low cost per user every month
  • Feature-rich telephone system with added functionalities
  • A flexible and scalable system with the ability to make or receive calls across the world

How hosted PBX works

Offered as a cloud solution, Hosted PBX features a non-physical telephone system that operates over a network which essentially is the cloud. The internet access needed to complete the calls is often supplied by your VoIP service provider thereby making internal calls free in most cases. Far-off employees can also receive and make calls via the office network as long as they have internet access wherever they are.

Additionally, the whole telephone system is managed by a third party who takes care of setup routines and infrastructural costs. This means you don’t need to worry about the general maintenance or costs of managing the system as long as your provider is reliable.

As we mentioned earlier, Hosted PBX connects the office telephone system to the wider public phone system. Now, this offers convenience and flexibility to both the business and its customers. While no hardware will be installed in your customers’ premises, they’ll still have access to features offered by the traditional system.

Hosted PBX vs. on-premise PBX – how do these two systems compare?

For starters, hosted PBX is fully managed by the service provider who houses all the equipment the system needs to operate. As for the On-premise PBX, PBX hardware is installed in the business premises usually in the server room. This hardware requires a server to connect the telephone system to the IP phones which are then connected to the internet. Alternately, SIP trunks can be used to route voice data through the internet thereby facilitating easier making of calls.

With Hosted PBX, your business pays a monthly flat-rate fee, fees for minutes used, and other overhead communication fees depending on your provider’s rates policy. The system only requires a phone plugged into the router to work with VoIP (the router provides an internet connection).

Features of Hosted PBX

Hosted PBX is a reliable, convenient, and cost-friendly telephony system, but which features facilitate these benefits? Learn more in this section where we describe a few top features of Hosted PBX:


If you’re like most businesses, you’ve probably employed a secretary or several to help you with welcoming visitors and ensuring you never miss out on important opportunities that may come by when you’re away from the office. An auto-attendant mimics this role sort of acting like a virtual secretary who covers for you whenever a call comes through. The feature which is often included in most PBX telephony systems allows you to design messages to be passed to the callers whenever you’re absent. You can craft both formal and informal messages depending on the image you want for your brand. And the best part? Most auto-attendants are able to forward calls to various departments to ensure customers have someone to speak to whenever they call.

Find/Follow Me

Find/Follow Me ensures you or your employees are always contactable irrespective of geographical location and the device you’re on. The feature allows you to set up a single number which then connects to any device or numbers on demand. With it, you can be mobile and still reachable whenever customers call the business number back in your office.

Queue Management

The queue management feature improves customer satisfaction by organising callers and ensuring they are attended to within the shortest possible time. In most cases, it can also be used to remind customer agents about the next call to attend to especially when they spend too much time on one. The good thing is that the caller on the queue cannot hear the reminder notification which is good for privacy. The queue time can be adjusted accordingly to cater for increased call volumes on special days or events.

Fax and Voice to Email

Faxes have been indispensable in business setups since time immemorial. Hosted PBX improves the function these office equipment play by automatically routing messages received on it to smart devices like desktops, smartphones, and laptops. This is important in helping you save the money you spend on purchasing reams of paper and regular ink cartridges needed to print fax messages.

In addition, voicemail messages can be auto-transcribed and emailed to specified business emails. This facilitates easy collection of details from the callers whenever they leave voice messages.

Other features

Do Not Disturb

For notifying the callers that you’re unavailable/unreachable to allow you to focus on a task in hand.

Hot Desking

Customer agents can share a number from various locations with only login details needed to use the telephone system.

Caller ID

Displays caller numbers and their names when stored in the memory or database of a phone.

Call Recording

This is a handy feature that helps you secure conversations you need for future reference.

Pros and cons of using hosted PBX


  • Reduced telephone expenses when you’re using a cloud-based virtual telephone system provider
  • Increased flexibility as you add functions only when and as they’re needed
  • Scalability as you only pay for what you use
  • Minimal installation costs as third parties provide necessary hardware and infrastructure


  • Call rates are dependent on your service provider
  • May attract maintenance costs in some cases

Factors to consider when migrating or setting up a VoIP system

Irrespective of the variant of VoIP you go for, the following are key factors you’ll need to consider before migrating.


As is evident, all VoIP systems require an internet connection to work. While you’re unlikely to labour finding internet sources today, not all possess the amount of bandwidth needed to hold a quality VoIP call. As such, it’s important that you set up your office in a location with not only numerous internet sources but also with enough bandwidth to cater for your needs.

Both Hosted PBX and SIP Trunks require a reliable Wi-Fi supplier or at least a 4th Generation SIM network. Unfortunately, these options can be hard to come by if you’re in a remote area with poor network reception.

Nature of hardware

You need to acquire a set of hardware that is compatible with VoIP if you’re to get the best out of this telephony system. So if you’re currently using outdated ones like phone lines and desk phones, you may want to start planning on how to modernise your system especially with the looming ISDN switch off. A good place to start is purchasing devices like mobile routers, Wi-Fi Extenders, tablets, and laptops for increased convenience. For software, consider investing in brands with powerful inbuilt firewalls to help you limit data loss or theft incidences.

Quality of service

What’s your estimated call volume in a month? Do you need HD audio in voice communications or will standard audio still nail it? Taking into consideration the quality of audio is vital especially if you’re to achieve great results doing tasks like auto-transcribing your calls.

Level of security

This factor is pretty much dependent on your preferred service vendor. A VoIP provider with highly secure and modern infrastructure is likely to guarantee more safety from fraud, data loss, and privacy breach.

Migration partner

With ISDN nearing its expiration, employing a migration partner who understands both the legacy and modern networks is vital. Here at Metrotech, we are ably-equipped and knowledgeable about system integration, mass data handling while we also offer high-tech telephone hardware to meet your exact telephony needs.

Check out our reliable VoIP services including SIP Trunks and Hosted Telephony. We also use the latest connectivity technologies such as Fibre, EFM, and FTTC to ensure you enjoy the clearest of calls and fast data transfer speeds.

An overview of Metrotech UK

Metrotech Solutions is a UK-based telecommunication firm with offices in Bolton. We’re an accredited provider involved in helping users of both ISDN and PSTN service successfully migrate to the newer VoIP telephony before BT’s ISDN switch-off finally takes place.

We offer modern and large-scale internet connectivity options including Fibre Leased Lines, EFM Connectivity and Generic Ethernet Access over the existing networks. In addition, we connect you to one of the two recommended VoIP technologies – hosted telephony (cloud-based/hosted PBX) or SIP Trunks – usable across the UK and beyond.

Why choose Metrotech for your VoIP installation?

Whether you’re just starting out or have an existing business, Metrotech is determined to help you get ready for the ISDN switch-off. This includes assisting you migrate successfully to VoIP if currently using legacy networks as well as helping you set up your first modern telephony system.

With an extensive experience which we’ve acquired over the years working with both large corporations and small-scale customers (see these case studies), there’s every reason to want to work with us. We are experts in hosted telephony and IP-Talk SIP Trunking with a fully-equipped in-house engineering team. If quality, reliability, and cost-saving are what you are after, we’re truly ready to exceed your expectations.

Not yet convinced? Here are a few more reasons to work with us:

A wide variety of connectivity options to choose from including wireless and Ethernet solutions.

  • Valuable experience in telecommunication and networking
  • Cost-effective VoIP solutions such as Hosted Telephony
  • Accredited experts in ISDN to VoIP migration
  • Specialists in communication add-ons including video conferencing assistants
  • Free inter-office calls
  • 24-7 support with a 4-hour fix time in case of a downtime
  • We use your existing PBX systems and retain your phone numbers

Two types of VoIP systems that Metrotech offers

IP-Talk SIP Trunks

This VoIP system is best known for its multi-layer security functions which help you control who reaches your phone line from the PSTN network. Besides, it offers increased flexibility since you can scale up or down the service charges depending on your present call volume.

IP-Talk SIP Trunks support both local calls in the UK and international ones at pocket-friendly prices. In addition, all inter-office calls are free as is the case with ISDN phone lines. Importantly, businesses with several branches can have their phone systems incorporated into WAN for free calls amongst them.

Hosted Telephony

This Hosted Telephony solution is available to all businesses regardless of their sizes. In it are all important features you probably need including call reports, call recording, queue management, auto-attendants, conference bridges, and hot desking. What’s more, Metrotech will retain your existing phone numbers which means minimal disruptions when communicating with your existing contacts.

The PBX system we offer you supports end-to-end communication across your various business locations. This boosts the overall privacy and security any time you’re communicating with the rest of the world.

Finally, we fully manage your system infrastructure, meaning you have zero maintenance costs to worry about. Besides, we are available all day long seven days a week in case you need support on anything including emergencies.

Let us connect your business to the future. Save money and enjoy improved telephony services with our VoIP technology!