Using Retell AI with a UK phone number: setup and UK GDPR checks

The two ways to connect a UK number

Retell’s custom telephony guide describes two options:

1. Elastic SIP trunking (recommended by Retell). You create a SIP trunk at your provider, point the number at it, and import the number into Retell. Inbound calls are sent to Retell’s SIP server, sip:sip.retellai.com. For outbound calls, Retell dials your provider’s termination URI.
2. Dial to a SIP URI. For providers without elastic SIP trunking, or more complex setups, the call is sent to a specific SIP URI.

Retell names Twilio, Telnyx and Vonage among supported providers, and has a step-by-step guide for Twilio.

Step by step with Twilio

Based on Retell’s Twilio guide and Twilio’s UK rules:

1. Get a UK number approved. Twilio’s UK regulatory guidelines require a regulatory bundle. For a business with a local or national number, Twilio asks for the business name, registration authority and number, website and a valid UK business address, plus a mobile number for an authorised representative that is not a Twilio or other CPaaS number.
2. Create an elastic SIP trunk in Twilio and set up termination (outbound) with either an IP allow list or a username and password.
3. Set origination (inbound) to sip:sip.retellai.com.
4. Assign your UK number to the trunk.
5. Import the number into Retell with the termination URI (and credentials, if used). Retell offers a Test SIP Connection button that checks the termination connection without placing a call.
6. Place real test calls in both directions, as Retell’s guide recommends, and check audio and routing.
7. If the agent will call abroad, enable the destination countries in Twilio’s Voice Geographic Permissions for Elastic SIP Trunking.

UK GDPR: settle these before go-live

Calls contain personal data: voices, phone numbers and whatever callers say. Before launch:

• Know where data goes. Retell’s compliance page says it does not currently operate services within the European Union. A Retell support reply on its community forum says all processing and storage take place in AWS US regions.
• Sign the contract. Retell offers a self-sign Data Processing Addendum at no extra fee, according to its compliance page, which says the DPA includes EU Standard Contractual Clauses. A Retell support reply on its forum says the self-sign version does not include SCCs and that a separate DPA can be requested. Ask Retell which version you are signing.
• Check the UK transfer mechanism. EU clauses on their own are not a UK transfer mechanism. The ICO’s guidance on international transfers explains the UK options, such as the International Data Transfer Agreement or the UK Addendum to the EU clauses, and the UK-US data bridge for certified US organisations. Retell’s compliance page says its DPA and SCCs are “required for processing personal data of EU/UK residents”, but it does not mention the UK Addendum or IDTA. Confirm with Retell which applies; we could not confirm this from its public pages.
• Set retention. By default Retell keeps call and chat data forever. You can set a per-agent retention period from 1 day to 2 years, and choose a storage mode: everything, everything except PII, or basic attributes only.
• Tell callers. Update your privacy notice and consider a short message at the start of the call saying it is an automated assistant and whether it is recorded.
• Outbound calls are different. Marketing calls are covered by PECR. See our AI receptionists guide for the ICO rules.

For the wider checklist, read Is AI safe with customer data? A UK GDPR checklist for SMEs.

What it costs

You pay Retell for usage (see Retell AI's pricing page) and your telephony provider for the number and call minutes. Prices change and are quoted in US dollars, so ask any agency for a total monthly estimate at your real call volumes. Our AI receptionists guide lists what UK providers publish.

Want this built for you? Browse UK voice agent agencies and ask each one how they handle UK numbers and data transfers.

This guide is general information, not legal advice.