It is the question that comes up last in every migration and should come up first. A copper line tells the emergency operator where it is. A SIP call does not — and what fills that gap is configuration you are responsible for getting right.
Why a copper line was easy
An analogue line is physically wired to one address. When it dialled 999, the operator saw that address automatically, because the line and the building were the same fact.
A SIP call carries no such fact. It can originate from a laptop in a hotel, a mobile app abroad, or a handset someone took home — and the network has no way of knowing which.
What fills the gap
The address held against your account and your numbers. When a 999 call goes out over SIP, that registered address is what the emergency operator is given.
Which makes it worth stating plainly: if that address is out of date, the ambulance goes to the wrong place. Not a compliance abstraction — a physical outcome.
Check it when anything movesNew office, new floor, a site closing, a batch of numbers reassigned to a different location. Each of those is a moment to re-check registered addresses. It is a five-minute task that nobody schedules and everybody assumes someone else did.
Multi-site is where it goes wrong
One account, several buildings, numbers pooled across all of them — and a single registered address for the lot. A 999 call from the Leeds office then presents the Manchester head office address.
If you run more than one site, addresses have to be held per number or per range, not per account. Ask your provider how they do it, and ask to see what is currently recorded rather than being told it is fine.
Remote and hybrid staff
A softphone on a laptop at home is the hardest case: the person may be anywhere, and the address on file is an office they are not in.
There is no configuration that solves this completely. What helps is telling people plainly — if you are working away from the office, dial 999 from your mobile, not from the softphone. A mobile knows roughly where it is. The softphone does not.
That sentence, in the induction pack and on the wiki, is worth more than any technical control.
Power is the other difference
An analogue line drew power from the exchange, so it worked during a power cut. VoIP does not: no power means no router, no handset and no call.
For most offices that is an accepted trade, but it should be an accepted one rather than an unnoticed one. Where it matters — a lone worker, a site with vulnerable people, a lift phone — the answer is usually a mobile kept charged for the purpose, or a small UPS on the router.
Test it. Properly, and once
You can verify emergency routing without tying up a real operator. Ask your provider how they want it tested; most can confirm what address a 999 call from a given number would present without placing the call at all.
Do it once when the system goes live, and again after any move. Then write down the date you did it.
The five-line checklist
- Every number has a registered address, and it is the current one
- Multi-site accounts hold addresses per site, not one for everything
- Remote staff have been told to use their mobile for 999
- Somewhere critical has a phone that works without mains power
- Someone owns re-checking all of the above when anything moves
Emergency routing and address registration are part of how we hand a system over.
See our migration service
