Putting a UK Phone Number on an AI Agent
This is a setup guide. Which number to use, the three ways to connect it, what the platforms will ask you for, and one question about 999 that you should settle before you move your main line anywhere.
No argument, no thesis. Prices and documentation were read on 5 September 2026 and this area moves, so check anything you are about to spend money on.
Step 1. Pick the number type
Four ranges matter for a UK business. The rules come from Ofcom's National Telephone Numbering Plan, version with effect from 22 April 2025.
| Range | What it is | Use it when |
|---|---|---|
| 01, 02 | Geographic | You want to look local. Default choice for most businesses |
| 033 | Non-geographic, national | You want national presence. Costs the caller the same as an 01 or 02 call |
| 080 | Freephone | Free to the caller, and you pay for inbound minutes |
| 084, 087 | Revenue-share | Almost never. See below |
Two details that catch people.
03 is not one range. The Plan designates 030 for public sector and not-for-profit bodies, 034 and 037 for numbers migrating from matching 084 and 087 numbers, and 033 as the general non-geographic range. An ordinary company should expect an 0333 number. A provider offering you an 0300 is outside the designation.
084 and 087 charge your callers. Ofcom caps the Service Charge at 5.833 pence per minute on 084 and 10.83 pence per minute on 087, both excluding VAT, and the caller also pays their own provider's Access Charge on top. For an inbound line meant to win business, this is the wrong instrument.
For a real GBP reference point rather than a US price list, Voipfone publishes geographic and 03 numbers at £3 a month excluding VAT, and 0800 at £5 a month excluding VAT plus 3p a minute inbound.
Can you have an 0161 number in Bristol?
Yes, and this is the most misunderstood rule in UK numbering. Paragraph B3.1.1 of the Numbering Plan requires a geographic number to be assigned to a subscriber physically in that area unless "the Subscriber has requested a Geographic Number that includes the relevant Geographic Area Code" and "charges for calls to that Geographic Number remain consistent with charges for calls to other Geographic Numbers that include the same Geographic Area Code".
So out-of-area use is permitted outright. There is no application to Ofcom and no exception to request, whatever some third-party summaries claim. Ofcom looked at this again in its March 2022 geographic numbering statement and decided to "continue to permit telecoms providers to offer out-of-area use of geographic numbers (where requested by the end user)".
One limit worth knowing: the same statement confirms providers "are not obliged to provide an out-of-area number if requested". It is lawful, not an entitlement. If one provider says no, another may say yes.
Step 2. Settle the 999 question before you move anything
Do this before you port your main line, not after.
Ofcom's General Condition A3 covers availability of services and access to emergency services. A3.4 requires providers to ensure "all End-Users can access Emergency Organisations by using the emergency call numbers '112' and '999' at no charge", and "End-User" includes businesses. A3.2(b) requires providers to take all necessary measures to ensure uninterrupted access to emergency organisations. A3.5 adds that accurate and reliable caller location must be available to the emergency organisation at the time the call is answered, including the full postal address for a fixed-location service.
That duty sits on your provider rather than on you. But it only helps if the route your calls actually take carries 999, and this is where the platforms diverge sharply.
| Platform | Documented UK emergency position |
|---|---|
| Twilio | Supports UK 999 and 112, but its own UK terms restrict emergency calling to two named products: Elastic SIP Trunking and Programmable Voice SIP Interfaces |
| Telnyx | Routes UK 999 and 112, plus 101, 111, 119 and 105, determining destination country from the caller ID |
| Vapi | Its phone-calling documentation says nothing about emergency calling at all |
| Retell | Same. Silence rather than a stated position |
Read the Twilio row carefully. "Twilio supports UK 999" does not mean a Twilio number pointed at a voice-AI platform has emergency access, because the emergency entitlement attaches to specific Twilio products. Silence in a platform's documentation is not permission to assume it works either.
Twilio's UK terms also push the caller-location duty onto you in absolute terms, and carry a warning worth repeating to any business moving off a traditional line: emergency calls may fail in a power cut or if broadband fails, so keep a second means of contacting emergency services.
This is not theoretical. Ofcom fined Vonage Business Limited £700,000 for breaching A3.2(b) after UK business customers on desk phones could not reach 999 from 13:35 on 23 October 2023 until 16:57 on 3 November 2023, eleven days and three hours. The cause was a routine software change whose test plan did not cover emergency calls, and it surfaced only because a customer complained three times. Vonage's residential customers were unaffected. Only business phones broke. That penalty reflected a settlement discount on a larger figure after Vonage admitted liability.
The practical answer for most small businesses: keep a working mobile or a separate line for emergencies, and do not make your AI-answered number the only way anyone in the building can call 999.
Step 3. Choose one of three routes
Route A, buy the number from the voice platform. Simplest, and unavailable in the UK on the two most common platforms. Vapi's free numbers are US-only, capped at five per account, and cannot dial out. Retell's managed numbers are US and Canada only, at $2 a month for US, $5 for US toll-free and $2 for Canada. For a UK business, Route A is not on the table with either.
Route B, bring a Twilio or Telnyx number. The normal path. You buy the number from the carrier, complete the UK regulatory requirements, and connect it to the agent with API credentials. This is what most UK deployments run on.
Route C, SIP trunk into an existing phone system. For businesses that already have a PBX and want the agent to answer some calls rather than all of them. Twilio's Elastic SIP Trunking uses regionalised termination URIs, with Dublin as the European edge, and SIP trunking requires an upgraded account rather than a trial one. It is also the route that carries emergency calling on Twilio, which may decide it for you.
What the carrier will ask for
The step that actually delays people is not technical. UK numbers require regulatory identity evidence: a regulatory bundle with proof of address, and for a geographic number, an address in the relevant area. Budget days rather than minutes, and start it before you plan the cutover.
On audio quality, expect narrowband
Twilio's Elastic SIP Trunking supports PCMU and PCMA by default, with G.729, Opus and AMR-NB in Limited Availability, and its documentation states those extra codecs "are not supported end to end". Telnyx documents genuine wideband support including G.722, Opus and AMR-WB. Retell lists PCMU, PCMA and G.722. Vapi's SIP documentation specifies no codecs.
Our inference, and it is inference: wideband will not survive to a UK PSTN caller whatever you configure, because no platform documents it surviving an interconnect and Telnyx's own example negotiates G.722 at 8 kHz. Configure for G.711 and design for narrowband audio, which is also why the names and postcodes problems are worse on a phone line than in your testing.
Step 4. Check the things that break around the port
Concurrency. Vapi includes 10 concurrent calls and charges $10 per additional line per month. Work out your busiest ten minutes before you pick a tier.
What else is on the line. If you are porting a traditional line, find out what else depends on it: broadband, an alarm system, a card terminal, a lift phone. These are the failures that surface a week later.
The PSTN switch-off. The UK is retiring the analogue network, and every business changing its phone setup now is doing so against that deadline. Get the current date from your provider rather than from an article, including ours, because it has moved before.
Number reputation. A recycled number arrives with whatever history it had. If it was previously used for outbound sales, it may already carry a spam label, and the remedy sits with the labelling companies rather than a regulator. That, and screening what reaches the agent, is covered in the screening ruleset.
What does not apply in the UK
Most published guidance on this is American, and three of its requirements do not exist here.
There is no 10DLC registration for UK numbers, that is a US carrier regime for application-to-person messaging. The UK does not operate STIR/SHAKEN call authentication; its spoofing mechanism is the Do Not Originate list. And FCC porting timescales have no bearing on a UK port, which runs under Ofcom's switching rules.
If a setup guide tells you to register a 10DLC campaign for your UK line, it was written for a different country.
Before you cut over
- Number type chosen, and if it is out-of-area, a provider that will actually issue it.
- Regulatory bundle submitted and approved.
- A written answer from your provider on whether 999 works on your route, and a second means of calling emergency services regardless.
- Everything else that shares the old line identified and rehomed.
- Concurrency sized for your busiest period.
- The number tested end to end, from a mobile, on the real route, before the old line is ceased.
Who holds the right to move that number if you later change supplier is a separate question, and it is not always the business whose name is on the invoice. That is in who owns your AI receptionist.