The UK PSTN switch-off: what actually changes for your phone lines

Every business with a copper phone line has now had a letter about the switch-off, and most of them are longer on urgency than on detail. Here is what is actually being turned off, what carries on working, and the order to do things in.

What is being switched off

Two things, and it helps to keep them apart. PSTN is the ordinary analogue line — the one a normal phone plugs into. ISDN is the digital successor businesses bought in blocks of channels, usually ISDN2 or ISDN30, to feed an on-site phone system.

Both are being retired. The copper is not being dug up; what is going away is the voice service delivered over it. From then on, voice runs over broadband or a dedicated circuit, as SIP.

What stops working, and what does not

Anything that dials over the voice path of a copper line needs attention. In practice that is a longer list than most people expect:

• Analogue extensions and ISDN channels feeding an old PBX
• Lift emergency phones and disabled refuge intercoms
• Fire and intruder alarm diallers that report over the line
• Door entry panels that ring out to a phone
• Card payment terminals still using dial-up as their fallback
• Franking machines, fax and some older building management kit

Broadband itself is not affected in the way the letters imply. If your line already carries broadband it carries on doing so; it is the voice service riding alongside it that changes.

Your phone system probably does not need replacing

This is where a lot of money gets spent unnecessarily. Any PBX that speaks SIP can be fed by a SIP trunk instead of ISDN, and most systems installed in the last fifteen years either speak SIP already or take a card that does.

An Asterisk platform is SIP-native, so for those the switch is a configuration change rather than a project. Even where the hardware genuinely is too old, moving the same system into the cloud is usually cheaper than buying a new one on the spot under time pressure.

Do this in the right order

1. Inventory every line. Not just the ones people ring in on — every line the building pays for, including the ones nobody remembers ordering. That lift phone is on the list.
2. List the numbers you must keep. These are the ones on your website, your vehicles and your customers' address books. Everything else is negotiable.
3. Check what the PBX supports. Make, model, firmware. The answer decides whether this is a configuration job or a replacement.
4. Size the connection. Concurrent calls, not headcount — see our note on how many channels you actually need.
5. Order SIP and test it in parallel. Run both for a fortnight with a handful of extensions on the new path.
6. Port the numbers last. Only once calls are proven on the new route.

The mistake that costs the most

Cancelling the old line before the numbers have moved. A ported number is transferred from a live line; cancel the line first and the number is released, at which point getting it back ranges from difficult to impossible.

Order the port, wait for the confirmed date, let it complete, then cease the old service. In that order, every time.

Leaving it late is the expensive option

The work is not hard. It is the timing that hurts: as regional stop-sell dates arrive, installers get busier and porting queues get longer, and a job that takes a fortnight when planned takes a month when rushed.

If you have not yet inventoried your lines, that is an afternoon's work and it is the step everything else depends on.