ISDN Switch Off - Everything you need to know

In 2025, BT will switch off its legacy networks, with traditional phone lines such as PSTN and ISDN becoming a thing of the past. But what does it mean for you and your business? In this post, we explain how it will affect your company communications and how you can adapt to the transition.

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What is the ISDN switch off?

Announced back in 2015, BT is conducting the "Big Switch Off", which refers to the gradual phasing out of BT's Integrated Services Digital Network (ISDN) and Public Switch Telephone Network (PSTN). Following September 2023, businesses will no longer be able to acquire PSTN or ISDN lines, and they will be totally redundant from 2025 with the switch-off taking place. The plan is that the ISDN turn-off will be completed by the end of December 2025.

This is not something that will only impact businesses as it is a change that will impact analogue phone lines, just like you might have at home. 

What are ISDN lines and PSTN? 

Public Switch Telephone Network (PSTN): This is what we know as the traditional landline, like that you might have at home. The PSTN system sends analogue voice calls over a circuit-switched phone line made of copper wire. 

Integrated Services Digital Network (ISDN): This is a telecommunication network that allows data to be transmitted as digitised signals, including sounds and images. Importantly for businesses, it allows for more than one call to take place simultaneously.

Why is it being switched off?

The simple reason BT decided to switch off ISDN lines is that this technology is not keeping up with the current times. ISDN lines and PSDN were introduced back in 1986, making this technology over 30 years old. With other emerging systems becoming more popular and cost-effective, they decided it did not make financial sense to keep improving the ISDN network. 

According to a study completed by Finance Online, by using VoIP technology to make international calls, your business can save a whopping 90%, with a total of 30% savings on your entire phone bill. Another study conducted by Microsoft stated that 82% of companies report cost savings when moving to the cloud.

BT understands that ISDN and PSDN offer little flexibility and are decreasing in popularity. It is becoming more transparent that IP technology is a better choice to meet future demands and business growth. 

ISDN Switch Off - Everything you need to know: Blog by T2k

What does the ISDN switch-off mean for my business?

If you have already transitioned from ISDN to internet protocol technology, then the switch-off will not affect your business. Ofcom reported that over two million businesses are still using ISDN connections. If you are one of those businesses still using the traditional ISDN or analogue telephone lines, then yes, the ISDN switch-off will affect you. 

As a company that still uses ISDN, does 2025 seem a long distance away? You may think that you have a long time to put off the switch to an alternative phone system before the inevitable occurs in just a couple of years, but 2025 is only a few short years away. As time is getting nearer, it is essential to start planning the transition to an alternative internet based communications system before the deadline sets upon us. 

Even though the deadline draws closer, there is no need to panic. The great news is that if you start planning the switch now, you still have plenty of time to figure out what you need to do before the deadline in December 2025. Your business must take time to research your options before investing in a new phone service to find a VoIP provider that meets your requirements and fits your budget.

What is ISDN going to be replaced with?

VoIP and SIP will be replacing ISDN. You can either opt for a cloud based phone system, such as Gamma Horizon, or 3CX which is a piece of software you could install anywhere, including your office premises. Deciding what you will need depends on your business communication requirements so we recommend you seek specialist advice. If you need direction, get in touch with T2k today for guidance from one of our professionals. 

What is VoIP?

VoIP is Voice over Internet Protocol and is a term used to describe a call made over the internet rather than a fixed-line system. 

What is SIP?

SIP is Session Initiation Protocol and is a signalling protocol that enables the Voice Over Internet Protocol. It manages multimedia system communication sessions (image, voice, video and messaging) in VoIP applications. 

For a more in-depth overview of these phone technologies, read our article: What is the difference between VoIP and SIP? 

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What to consider before making the switch

Switching from ISDN to IP technology is quite straightforward, however, there are some things that you need to consider before taking the plunge and investing in new technology. The following are some of the things to keep in mind when researching your options. 


What features will you need to ensure smooth-running business operations?

You need to have a clear idea of what features your business requires from a phone system as some services offer add-ons that others cannot. You do not want to choose something that will become inefficient or ineffective for your business now or down the line. Talk to your provider before making the switch to ensure that they have all the components needed for your specific business needs. 

For example, if your company needs call recording, then 3CX or Gamma Horizon would be great condensers as they include this feature. But some phone systems, such as Phoneline+, do not have this option available. 

Unsure what features you will require? According to a study conducted by Finance Online, the top VoIP features and benefits for small businesses are find me/follow me (77%), handling phone calls (67%), conference calling (65%), managing messages (63%), Mobile IP PBX (58%) and remote work (57%).

Your existing system 

If you are a small business, such as a hairdresser, beauty salon or shopkeeper, you’ll most likely be using an analogue phone line. The best alternative for you could be a service such as Phoneline+, as it is a fully hosted solution that will ease management and lower cost. 

However, if you are a larger business with an on-site PBX, you might prefer to keep your existing equipment and update it to adapt to the change. If you are considering this option, check that your current system is IP compatible. If it is not, we recommend that you consider a hosted solution to avoid significant costs. 

User number 

How many users do you intend to use on the cloud-based phone system? Different phone systems are designed for various user numbers. 

  • If you are a smaller-sized business with 1 - 3 users, opting for a simpler system might work best for your business, such as Phoneline+.
  • If you are a medium-sized business with a few more users, look at a cloud phone system with more features will be wise, such as Gamma Horizon
  • If you are a larger-sized business with 20+ users, then you are best opting for a more cost-effective phone system that is flexible, scalable and feature-rich, such as 3CX.
ISDN Switch Off - Everything you need to know: Blog by T2k

How to make the switch from ISDN to VoIP

It can be confusing to figure out how to make the switch from ISDN to VoIP. To help, we have outlined a step-by-step guide. Follow these simple steps to successfully migrate your ISDN system to a VoIP phone network. 

Audit your current system

Look at your current setup and assess your internet connection. Check features and procedures, including phone line numbers, extensions and worker locations. Assessing this will determine what needs you will require with a new system. 

Create a plan 

With a clear vision of your telephone requirements, you need to identify what you want from your new system. Is it to reduce costs, increase connectivity or a combination of both? Start plotting the details necessary to achieve those goals (such as hardware and additional features needed). Your communication strategy is vital as it will determine the VoIP provider you choose.

Consider your hardware and choose a VoIP provider

Choose a provider that offers high-quality security and support. You will also want to opt for a service that delivers the correct features, add-ons and hardware your business requires. Refer back to 'what to consider before making the switch' for more information on elements and hardware to look into when making the transition. 

Test, familiarise and use

Once your VoIP system is in place, you will want to test the call quality when there is both a lot and little network activity. If any problem occurs, you can contact your VoIP provider to fix any issues.

Familiarise yourself with the new features and train your team to make the best use of the new system.  

Need help making the switch? T2k are here to help

It can feel daunting as a business to prepare for a switch in technology, especially if you do not know anything about the systems or are unsure about what you may need to best benefit your business. We are here to help make your decision-making process easier with jargon-free conversation on the right system that will best suit your business. Get in contact with T2k today for our specialist advice on how to make the switch and assess different VoIP systems to find the best one to fit your needs. 

Lee Clarke
Sales Director

Having worked for T2k for nearly 25 years, it's fair to say that Lee is an expert when it comes to all things telephony and business communications. Overseeing the commercial side of the business, he has helped the company evolve and grow through the decades. In recent years, and with the advent of VoIP and hosted telephony, Lee has made sure that T2k is at the forefront of technological developments. With a firm interest in helping businesses navigate the world of telecoms, Lee is responsible for the majority of the content on this website.

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