The UK PSTN switch-off: what it means for your phone system

The UK's Public Switched Telephone Network — the copper analogue network that has carried voice calls for over a century — is being retired. Alongside it go ISDN lines, the digital circuits that most legacy PBX systems depend on. If your organisation still routes calls over ISDN, this is not a distant infrastructure story: it is a hard deadline for your telephony estate.

What is actually being switched off

Openreach has stopped selling new PSTN and ISDN services, and the network itself is being withdrawn, with the national switch-off now targeted for the end of January 2027. After that point, analogue lines and ISDN circuits simply stop working. Everything that relies on them — desk phones behind an on-premises PBX, fax lines, some alarm and lift lines — needs an all-IP replacement.

Why this matters more for larger organisations

• More circuits, more dependencies. A 500-user organisation typically has ISDN30 circuits at multiple sites, direct dial number ranges, and contractual commitments that all need mapping before migration.
• Number porting takes planning. Large number ranges need to be ported to a new provider in a controlled sequence to avoid service gaps.
• Edge cases hide in plain sight. Door entry systems, franking machines, emergency lines in lifts — these analogue dependencies are often discovered late.

Why Teams Voice is the natural destination

For organisations already running Microsoft 365, moving telephony into Microsoft Teams removes the PBX entirely rather than replacing it like-for-like. Calls travel over your existing internet connectivity, users keep one application for calls, meetings and chat, and PSTN access is delivered through Operator Connect or Direct Routing rather than physical circuits.

How to approach the deadline

• Audit first. Inventory every line, circuit and analogue dependency across all sites before choosing a replacement.
• Sequence the migration. Pilot with one site or department, validate call quality and call flows, then roll out in waves.
• Do not leave porting to the end. Number porting lead times are the most common cause of slipped migration dates.

The switch-off is fixed; the only variable is whether your organisation migrates on its own schedule or under deadline pressure. If you would like help scoping what the switch-off means for your estate, get in touch for an initial conversation.