Custom SIP Trunk not sending REGISTER packets (UDP)

Support Help
1

Hi Support/Dev Team,

I’m hitting a critical SIP Trunk bug. Retell is NOT sending REGISTER packets to my European provider (sip.fcn.pl).

My provider’s network admins checked their firewall/DROP logs and confirmed zero incoming packets from Retell’s IP ranges (18.98.16.120/30, 143.223.88.0/21, 161.115.160.0/19).

Everything on my Retell dashboard is configured perfectly (sip.fcn.pl:5060, UDP, correct login/pass - see screenshot).

I am aware that Retell works perfectly with providers like Telnyx (which often use IP-auth/direct routing), but FCN strictly requires explicit SIP REGISTER packets with credentials, which your system currently does not seem to be sending out to their domain.

Could an engineer please check the outbound SIP logs for my workspace and fix the REGISTER mechanism?

Workspace name: maciej szymon maciorski’s

2

Hey @maciorskimaciej Can you share relevant call Ids?

3

Hey Shaw,

There are no Call IDs because the calls never reach Retell’s infrastructure. Since Retell isn’t sending the periodic SIP REGISTER packets, my provider (FCN) considers the trunk offline and drops all inbound calls on their end.

Could you please check your outbound SIP registration logs (not call logs) for the trunk pointing to sip.fcn.pl? We just need to see why Retell isn’t attempting to authenticate/register with the provider.

Thanks!

4

Hey @maciorskimaciej Can you share your Agent ID?

5

Hey Shaw,

Sure, here is an Agent ID from my workspace: agent_398fa43ff64e437888e46c3f11

I have several agents, but just to reiterate: this issue happens before the routing even reaches the agent layer. FCN drops the connection at the firewall because Retell’s SIP Trunk isn’t sending the periodic UDP REGISTER packets with credentials to sip.fcn.pl to keep the trunk alive.

Let me know what you find in the outbound registration logs!

Thanks.

6

Hey @maciorskimaciej I share this with the team to look.

7

Hello @maciorskimaciej

Thank you for the detailed write-up and for confirming the firewall findings with FCN’s network team. This helps clarify what is happening.

The issue is not a bug. Our SIP implementation does not send REGISTER packets. When a call is placed outbound, our system sends a SIP INVITE directly to the termination URI you configured (sip.fcn.pl:5060). If FCN challenges with a 401 or 407, we respond inline with the username and password stored in your trunk config. There is no periodic REGISTER keepalive in our SIP stack.

This means FCN will not receive any packets from our egress ranges (18.98.16.120/30, 143.223.88.0/21, 161.115.160.0/19) until an actual outbound call is placed. Their firewall sees no traffic in between calls because none is sent. This is expected behavior, not a missing packet.

To make this trunk work with FCN, you have a few options:

  1. Ask FCN to configure your trunk for IP authentication. They would allowlist our egress CIDRs and accept inbound INVITEs with inline Digest auth, no REGISTER required. Most carriers that default to REGISTER-based trunks can offer this on request.

  2. Use a SIP proxy between our system and FCN. A registration-capable proxy such as Kamailio, OpenSIPS, or FreeSWITCH would register with FCN on your behalf while presenting a static IP interface to our system.

  3. For receiving inbound calls from your FCN DID, FCN would need to send INVITEs for that number to sip:sip.retellai.com, and the DID would need to be imported into your workspace. More detail here: https://docs.retellai.com/deploy/custom-telephony

Please let us know if you have any questions after coordinating with FCN.

Thank You