Inbound Custom Telephony returns SIP 486 USER_BUSY for AU number (+61) only

Support Help
1

We are reporting a deterministic inbound Custom Telephony issue.

Summary:

  • Australian geographic number (+61) imported and bound to inbound_agent_id
  • Inbound SIP INVITEs reach Retell and are immediately rejected with SIP 486 USER_BUSY
  • Call duration ~1–2 seconds
  • Behavior is deterministic and number-specific (AU only)

Control test:

  • Same PBX, same SIP trunk, same Retell agent
  • Spanish numbers work perfectly
  • Swapping only the phone number reproduces / resolves the issue instantly

What has been verified:

  • Carrier and PBX routing confirmed
  • Number import and inbound_agent_id binding confirmed
  • No concurrency, saturation, or rate limiting
  • SIP 486 is generated by Retell

Relevant precedent:

This appears consistent with a cached inbound SIP routing / fallback state or internal validation issue.

Request:
Please check and reset inbound SIP routing / fallback state for this AU number.

We can provide:

  • Org ID
  • Phone number
  • Exact timestamps
  • Full SIP traces
2

Based on the SIP 486 USER_BUSY error you’re encountering with your Australian number, this indicates a “Busy Here” SIP response from Retell’s side(1). When inbound calls fail to connect with this error, the disconnection_reason field will show dial_busy, which means the number dialed is busy(2).

For custom telephony integrations, when inbound calls aren’t connecting properly, you should:

Check Call Detailed Logs
Review the call detailed log in your Retell dashboard, which contains the SIP error code and error message that should help identify the root cause(1).

Verify Number Configuration
Double check your configuration and ensure you imported the correct information(3). If the configuration is not correct, delete the imported number and re-import it(3).

Ensure your phone number has an inbound_agent_id set correctly(1). You can update your phone number configuration using:

const phoneNumberResponse = await client.phoneNumber.update('+14157774444', {
  inbound_agent_id: 'oBeDLoLOeuAbiuaMFXRtDOLriTJ5tSxD',
});

(4)

If inbound_agent_id is set to null, the number will not accept inbound calls(4).

Transport Protocol
Verify your transport protocol configuration. Retell’s SIP server supports TCP, UDP, and TLS(5). For TCP, the SIP server URL needs to be sip:sip.retellai.com;transport=tcp(5).

IP Whitelisting
Ensure you’ve whitelisted Retell’s IP block 18.98.16.120/30 for all regions(5).

Since you’ve verified carrier routing and the number binding, and this is deterministic for the AU number only, you should reach out to Retell support with your org ID, phone number, exact timestamps, and full SIP traces through these channels:

(5), (3)