Tutorial
Retell AI Twilio Setup: Complete Step-by-Step Guide (2026)
How to connect Retell AI to Twilio in under 30 minutes. Buy a number, import the SIP trunk, wire inbound + outbound, and ship your first call.
Twilio is the path of least resistance for Retell AI telephony. SignalWire, Telnyx and Plivo work too, but Twilio's docs and support cycle are still the fastest. Total setup time once your accounts exist: about 20 minutes.
Buy a Twilio number
In Twilio Console → Phone Numbers → Buy a Number, pick a local DID in the area code your callers expect. Voice capability is the only requirement. Keep the number tab open — you'll need the E.164 string in step 4.
Create an Elastic SIP Trunk
Twilio Console → Elastic SIP Trunking → Create new trunk. Name it 'retell-prod'. Under Termination, set a unique URI like retell-prod.pstn.twilio.com. Under Origination, add the SIP URI Retell gives you in step 3. Enable Symmetric RTP.
Connect the trunk to Retell
In Retell dashboard → Phone Numbers → Import via SIP Trunk. Paste your Twilio Termination URI and create credentials (Twilio → Trunk → Authentication → Credential Lists). Retell will hand back an Origination SIP URI — paste that into Twilio's Origination block.
Assign the number to the trunk
Back in Twilio Phone Numbers, open the number you bought, scroll to Voice Configuration, and set 'Connect to' → SIP Trunk → retell-prod. Save. Inbound calls now flow Twilio → SIP → Retell agent.
Test inbound
Pick up your phone, dial the Twilio number. You should hear your Retell agent greet you. If you hear silence or a Twilio error, 90% of the time it's a credential mismatch or the Origination URI is wrong — check both.
Enable outbound dialing
Retell dashboard → Agent → Outbound. Select your Twilio trunk as the origination source. Test with a single outbound call to your own mobile before loading a list. Match caller ID to a verified Twilio number to avoid spam-likely flags.
Set up call recording + webhooks
In Retell → Webhooks, point call_ended at your server. Store recordings, transcripts and metadata. This is non-optional for QA and prompt iteration — you cannot improve an agent you cannot replay.
Common gotchas
- Don't forget the credential list on the Twilio side — Retell origination URIs are auth'd.
- Set Twilio Voice Geographic Permissions to include the countries you'll call.
- Enable Twilio Voice Insights — it'll save you a day of debugging audio quality.
- For high outbound volume, get a verified caller ID and STIR/SHAKEN attestation.
Stuck on a SIP error? A Retell AI consultant can usually unblock the trunk in 15 minutes — or hire a full Retell AI agency to ship the whole telephony stack.
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