Retell
Put a Retell AI agent on a Zernio number: SIP forwarding or full number import via SIP trunk
Retell
There are two ways to connect a Zernio number to a Retell AI agent, and they work exactly like the ElevenLabs integration — the number, its countries and KYC, SMS, and WhatsApp stay with Zernio; what changes is who handles the calls.
Retell's SIP ingest is sip.retellai.com on port 5060 over TCP.
Forward inbound calls
Set the number's forward destination to Retell's SIP address, using the number in E.164 as the identifier:
curl -X POST "https://zernio.com/api/v1/phone-numbers/PHONE_NUMBER_ID/voice" \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{"forwardTo": "sip:+34911234567@sip.retellai.com:5060;transport=tcp"}'In Retell, add the number under Phone Numbers → Connect to your number via SIP trunking with the same E.164 (keep the leading + identical on both sides) and assign your agent.
SIP trunk
A SIP trunk takes Zernio out of the call path: Retell receives the number's inbound calls directly, and the agent can place outbound calls through Zernio presenting the number as caller ID.
Create the trunk
From the dashboard (Numbers → SIP, provider preset Retell) or the API:
curl -X POST "https://zernio.com/api/v1/phone-numbers/sip-trunks" \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{"label": "Retell", "sipHost": "sip.retellai.com", "sipPort": 5060, "transport": "tcp"}'Store the digestPassword from the response — it is shown only once (rotate later if lost).
Configure Retell
In Retell's SIP trunking setup, point outbound at the trunk's termination URI (sip.telnyx.com) with the digest username and password from step 1, and register the number in E.164 with the leading +.
Attach the number
Attach from the dashboard trunk page, or POST /v1/phone-numbers/{id}/sip-trunk with the trunkId. Detach any time — Zernio routing and voice features come back immediately.
The ElevenLabs page's technical FAQ — codecs, authentication, caller ID, limits, countries — applies to any trunk destination, Retell included. One difference: Retell's ingest is TCP, so signaling is not TLS-encrypted and media is not SRTP on the inbound leg.