In short: ISDN replacement options are the ways to move off ISDN2 and ISDN30 before Openreach closes them on 31 January 2027. You can keep your PBX and connect it with SIP trunks, move to a hosted phone system such as Forever UC, hosted 3CX or Webex Calling, move to Microsoft Teams Phone, or choose BT Cloud Voice.
What ISDN does today and why it is going
ISDN is a digital phone service delivered over Openreach's copper network. ISDN2 gives two channels, so two calls at once, and is common in small offices. ISDN30 is delivered in bundles of channels up to 30 per bearer and usually carries a range of direct-dial (DDI) numbers into an on-site PBX.
Openreach stopped selling new analogue and ISDN lines (its WLR products) across the UK on 5 September 2023, and its withdrawal of analogue and ISDN services is aligned with the PSTN switch-off date of 31 January 2027. Emergency Voice Access, Openreach's temporary fallback, is a voice-only service, so do not count on it to keep an ISDN phone system running. Your numbers do not have to go with it: they can be ported to whichever option you choose. Our PSTN switch-off checklist covers the full migration plan.
The four options side by side
Each option is a sound choice for a particular starting point. None is right for everyone.
| Keep the PBX on SIP trunks | Hosted phone system | Microsoft Teams Phone | BT Cloud Voice | |
|---|---|---|---|---|
| What changes | Only the lines. The PBX stays and connects over the internet | The PBX moves to a data centre and is run for you | Calls move into the Teams app, with a PSTN connection added | BT runs the phone system and lines as one service |
| Existing handsets | Keep them | SIP desk phones may be reusable if supported; proprietary PBX handsets usually are not | Compatible SIP phones can sign in through Microsoft's SIP Gateway; otherwise Teams devices or the app | BT supplies compatible handsets |
| Numbers | Ported to the SIP provider | Ported to the hosted provider | Ported to Microsoft, an operator or a Direct Routing provider | Ported to BT |
| Features | Whatever your PBX already does | Platform features, apps for mobiles and laptops, and regular updates | Teams calling, auto attendants and call queues, with chat and meetings | BT's cloud features and apps |
| Budget profile | Lowest change now, but the PBX still needs support and eventual replacement | Monthly or annual licences with little or no hardware | Per-user Microsoft licences, often alongside Microsoft 365 | Per-user monthly service from BT |
| Good fit when | The PBX is supported, has SIP or a gateway, and still meets your needs | You want a managed system and flexible working | Your organisation already runs on Microsoft 365 | You want BT as the single supplier for lines, broadband and phones |
None of these options has a published price that would apply to you without a survey, so ask us for a quote and we will price the options you shortlist side by side.
Option 1: keep your PBX and add SIP trunks
If your PBX is still supported by its maker and has a SIP interface, or can take a gateway that converts ISDN to SIP, SIP trunks are the least disruptive change. Staff keep their handsets and the call flows they know, and only the connection to the outside world changes.
Size the trunk on peak simultaneous external calls rather than simply copying your old ISDN30 channel count, because many ISDN bundles were bought with spare capacity. Our guide to how many SIP channels you need explains the sums. The trade-off is that the PBX itself still needs maintenance, security updates and, eventually, replacement. If you would rather keep 3CX on your own server, see hosted 3CX, which also runs on-premise.
Option 2: move to a hosted phone system
A hosted phone system moves the PBX into a data centre. There is no box on site to maintain, and users get desk phones, mobile apps and softphones on the same number. The platforms we supply include:
- Forever UC, Forever Group's own app-first service built on Cisco BroadSoft technology and hosted on the BT Wholesale network in the UK
- Hosted 3CX, licensed per system by simultaneous calls rather than per user
- Webex Calling from Cisco, which suits organisations already using Webex or Cisco networking
The hosted PBX page explains how your existing extensions, groups and menus move across.
Option 3: Microsoft Teams Phone, or option 4: BT Cloud Voice
Microsoft Teams Phone suits organisations whose staff already work in Teams. You add Teams Phone licences and choose how Teams connects to the phone network: Microsoft Calling Plans, Operator Connect or Direct Routing. Forever Teams is our Direct Routing service that keeps your existing numbers. If you want to keep an existing PBX while moving users to Teams, Call2Teams can link the two.
BT Cloud Voice suits organisations that want BT as one supplier for lines, broadband and phones. Our sister company Forever Communications Limited covers it in depth on ISDN replacement with BT Cloud Voice, and our BT Cloud Voice overview explains where it fits among the other platforms.
Questions to answer before you choose
- How old is the PBX, is it still supported, and when does its maintenance contract end?
- How many calls are in progress at your busiest moment, and does that include queues and conference calls?
- Which DDI numbers are in use, and are any printed on signage, vehicles or marketing?
- Do alarms, lifts, door entry or card terminals share the ISDN or analogue lines?
- Is the internet connection at each site good enough for voice, and is there a back-up path?
- Do staff need to take calls away from their desks, at home or on a mobile?
Our number porting guide explains how to keep every number. When you are ready, contact us and we will compare the options for your sites.
Why choose your cloud phone system with Forever Systems
We are vendor-neutral. Forever Group supplies and supports several cloud phone platforms, so we recommend the one that fits how your organisation works rather than the one we happen to sell.
- Side-by-side comparison of hosted 3CX, Microsoft Teams Phone, Forever UC, Forever Teams and Webex Calling for your users
- Contact centre and call recording options matched to your call volumes and CRM
- Number porting, handsets and set-up handled for you, including the move off analogue and ISDN lines
- Connectivity checked so call quality is right from day one
- UK support for moves, adds, changes and faults
- For BT Cloud Voice, see our BT Business Partner site cloudvoice.cloud
We work with businesses, schools, charities and public sector organisations. We do not sell consumer phone services.
Want a vendor-neutral cloud phone system comparison and quote?
Tell Forever Systems what you need and we will come back to you by email with options, not a hard sell.
Contact usFrequently asked questions
What is replacing ISDN in the UK?
ISDN is being replaced by internet-based (IP) voice. In practice that means SIP trunks into your existing phone system, a hosted cloud phone system, Microsoft Teams Phone, or a carrier service such as BT Cloud Voice. Openreach is withdrawing ISDN with the PSTN switch-off on 31 January 2027.
Can I keep my ISDN30 numbers when I switch?
Yes. Your numbers, including DDI ranges, can be ported to the SIP, hosted, Teams or BT service you choose. Keep the ISDN service running until the port completes, because cancelled numbers cannot be ported.
Can my existing phone system work without ISDN?
Often, yes. If the PBX is supported and has a SIP interface, or can use a gateway, SIP trunks can replace the ISDN lines. If it cannot, or it is near the end of its life, moving to a hosted system or Teams Phone is usually simpler.
How many SIP channels replace an ISDN30?
Not necessarily the same number. Size SIP channels on your peak simultaneous external calls, plus headroom, rather than copying the old ISDN30 channel count, which often included spare capacity.