AI Voice · Retell AI

Bind a phone number to your Retell AI agent (inbound)

Retell AI supports BYOC SIP trunking. This tutorial provisions a DIDHub trunk, registers it with Retell, and routes a DID so your Retell agent answers real phone calls.

Step 1: Create a DIDHub SIP trunk

# Dashboard → SIP Trunks → New trunk
Name      : retell-prod
Mode      : SIP digest auth
Username  : retell_b7e2
Password  : "<auto>"
Region    : us-east, us-west
Inbound   : enabled
Outbound  : enabled (for outbound campaigns)

Step 2: Register the trunk in Retell

curl -X POST https://api.retellai.com/create-phone-number \
  -H "Authorization: Bearer $RETELL_API_KEY" \
  -d '{
    "phone_number": "+15551234567",
    "termination_uri": "sip:sip.didhub.io",
    "sip_trunk_auth_username": "retell_b7e2",
    "sip_trunk_auth_password": "<PASSWORD>",
    "inbound_agent_id": "agent_abc123"
  }'

Step 3: Bind in DIDHub

Numbers → +1 555 123 4567 → Routing → SIP Trunk: retell-prod. Save.

Step 4: Test inbound

Call the DID. Retell agent should pick up within 1 ring. Logs are visible in both Retell dashboard and DIDHub trunk monitor.

Common errors

• "trunk_auth_failed": username or password mismatch — copy from the DIDHub trunk detail page exactly.
• 404 on phone-number assignment: the DID must be active in DIDHub before Retell registration.
• Caller hears silence for 8 seconds then RTP arrives: set codec preference to PCMU first in the DIDHub trunk — Retell prefers G.711.

Outbound (optional)

curl -X POST https://api.retellai.com/create-phone-call \
  -H "Authorization: Bearer $RETELL_API_KEY" \
  -d '{
    "from_number": "+15551234567",
    "to_number": "+15557654321",
    "agent_id": "agent_abc123"
  }'