Can You Keep Your Existing Phone System After the PSTN Switch-Off?

Yes, you can keep your existing phone system after the PSTN switch-off on 31 January 2027, but only if it can connect over the internet using SIP trunks. A system that relies on ISDN or analogue lines will lose its outside calls unless it's fitted for SIP or replaced.

Three checks decide it: can the phone system accept SIP, does your maintainer still support it, and can your internet connection carry the calls. Order matters, because new ISDN lines are already off sale.

How do you tell whether your phone system can use SIP trunks?

1. Find the make and model of the main unit. It's usually on the front panel or an engineering label.
2. Ask your maintainer whether the system supports SIP trunks, and whether that needs a software licence or an extra card.
3. Check how old the software is. Out-of-support software is a good reason to price a replacement alongside the upgrade.
4. Run the PSTN switch-off checker to see what your exchange area and lines mean for timing.

Some older systems only take ISDN or analogue cards. A gateway can sometimes bridge the gap, but it adds cost and another thing to go wrong, so it's worth comparing against a new system rather than treating it as free.

What are your options?

For a fuller picture of what SIP trunks are, read What is a SIP trunk and do you need one?

What does moving to SIP trunks involve?

1. Audit your lines. List every ISDN or analogue line and what it serves. Count the most calls you take at once, not the number of staff, because trunks are sized on simultaneous calls.
2. Check your bandwidth. Allow about 100 Kbps for each call in each direction, and ideally keep voice away from heavy downloads.
3. Order the trunks and plan your numbers. Number transfers take time, so see how long it takes to port a business number and start early.
4. Configure and test. Your maintainer sets up the phone system to use the trunks, and you test inbound, outbound and out-of-hours routing.
5. Switch over and keep a fallback. Leave the old lines in place until the new ones are proven.

What else sits on your phone line?

Alarms, lift phones, card machines and fax lines often share the same copper. Before you cancel anything, work through the PSTN switch-off checklist and ask each supplier what their equipment needs.

How much will it cost?

That depends on your system, your number of lines and the internet connection, so T2K quotes each one individually. If you decide a hosted system is simpler, per-user prices are published on the system pages, for example Gamma Horizon Essential at £12.95 per user per month excluding VAT.

Frequently asked questions

What happens to my phone system if I do nothing?

The system will still power up, but calls over ISDN or analogue lines stop working once the lines go. Internal calls would carry on, which can hide the problem until a customer can't get through.

Can I keep my phone numbers?

Yes. Numbers can usually be ported to a SIP provider or a hosted system, and T2K handles the paperwork. Start early, because transfers rarely happen overnight.

Will I need new handsets?

Not always. If your system accepts SIP trunks, the handsets can stay. If you move to a hosted or 3CX system, you may need IP handsets or a softphone.

Is it worth keeping an old phone system?

Sometimes. If it's well supported and does what you need, trunks are the lowest-disruption route. If spare parts are scarce or the software is out of support, price a replacement alongside it.

Can T2K help if we're in Dorset or Hampshire?

T2K is based in Bournemouth, so firms in Poole, Christchurch, Southampton, Winchester and across Dorset and Hampshire can talk to a local team.