Inbound calls to connected telnyx number always returns user busy

Support Help
1

The number makes outbound calls fine, however whenever I try to call the number it always rings once and then returns user busy. I have gone over all the settings 100 times and everything seems to be right.

Telnyx shows that the invite is sent to retell but then it returns a 486 error code. The number is definitely attached to an agent and I have tried it on multiple agents

Inbound Custom Telephony returns SIP 486 USER_BUSY for AU number (+61) only
2

Based on the 486 error code you’re seeing from Telnyx, this indicates a “Busy Here” SIP response from Retell’s side(1). The error means Retell’s SIP endpoint is rejecting the inbound call(1).

Here are the key things to check:

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).

Common Disconnection Reasons
When inbound calls fail to connect, the disconnection_reason field will show one of these values(2):

  • dial_busy - The number dialed is busy(2)
  • dial_failed - Dialing failed with no or unknown SIP error code(2)
  • telephony_provider_permission_denied - SIP trunk credentials are not authenticated(2)
  • telephony_provider_unavailable - The telephony provider is unavailable(2)
  • sip_routing_error - SIP routing is going over too many hops or is in a loop(2)

Verify Configuration
For custom telephony (like Telnyx):

  1. Double check your configuration and ensure you imported the correct information(3)
  2. If the configuration is incorrect, delete the imported number and re-import it(3)
  3. Check with Telnyx to see what error they’re seeing on their side(3)

The 486 response specifically suggests Retell’s endpoint is either busy, unavailable, or there’s a configuration mismatch between Telnyx and Retell(1). Since outbound works fine, the issue is likely with how the inbound routing is configured between Telnyx and Retell.

3

The attempted call doesn’t even show up on the call logs on retell’s side

4

If the inbound call attempt isn’t showing up in Retell’s call logs at all, this indicates the call isn’t reaching Retell’s system - the issue is likely with your Telnyx-to-Retell routing configuration.

Key Configuration Steps for Custom Telephony

When using imported numbers from Telnyx, you need to verify:

  1. Check your imported number configuration - Double check that you imported the correct information(1)
  2. Verify the inbound agent binding - Ensure your phone number has an inbound_agent_id set(2)

You can update your phone number configuration using:

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

(2)

Important Notes About Inbound Configuration

  • If inbound_agent_id is set to null, the number will not accept inbound calls(2)
  • The number will automatically use the bound agent when receiving inbound calls(2)

Troubleshooting Steps

Since the call isn’t reaching Retell at all:

  1. If the configuration is not correct, delete the imported number and re-import it(1)
  2. Check with Telnyx to verify their routing is correctly pointing to Retell’s SIP endpoint(1)
  3. Verify the SIP trunk authentication credentials are correct(1)

The fact that outbound calls work but inbound calls don’t even appear in logs strongly suggests the Telnyx inbound routing isn’t properly configured to reach Retell’s infrastructure.

5

Hello @William_Harrowsmith

Can you provide the PCAP file from telnyx