Unable to establish a SIP connection between our PBX and Retell (sip.retellai.com)

Support Help
1

Our PBX engineer cannot establish a SIP connection or receive any SIP response from sip.retellai.com. He also tried connecting directly to the resolved IP address with the same result. Any suggestions would be appreciated.

2

Hello @j3385mac Here’s what to check:

  1. Whitelist Retell’s IP blocks on your PBX/firewall — traffic must be allowed from:

    • 18.98.16.120/30
    • 3.42.144.0/23
    • 153.57.128.0/18
    • 143.223.88.0/21 (certain US traffic)
    • 161.115.160.0/19 (certain US traffic)

  2. Open the correct ports: TCP/UDP 5060 for SIP signaling (or TCP 5061 for TLS).

  3. Specify transport explicitly in the SIP URI, e.g. sip:sip.retellai.com;transport=tcp — TCP is recommended.

If connecting directly to the resolved IP still yields no response, the issue is almost certainly a firewall blocking outbound SIP from your PBX side.

Could you please check these settings and refer to the relevant documentation (Connect Retell voice agents to your telephony provider) , (Receive inbound calls with Retell agents).

Let us know if the issue persists after reviewing them.

Thank You

3

Thanks for the quick response Shah. I’ll let you know if I have any other questions.

4

Shah this is the response from the PBX engineer. Are you able to help me confirm my configuration is correct. V2 is the current live version. The hotels inbound call main number is 18313754500.

Jason,

It’s not a firewall issue, as we are not blocking any of these Retell AI IP addresses. Port 5060 is always open, as that is our standard SIP port.

I’ve looked over the documentation provided by Retell AI and other reports of users trying this exact setup. I have created the sip trunk to the specifications provided, but calls routed to it are not being answered by Retell AI. I have tried using both UDP (our standard transport) and TCP - both getting the same result. Please check the config on your side to make sure the call can be routed.

  • Import the phone number 18313754500 into Retell via SIP trunking
  • Set termination URI to c8313754500.callingcloud2.net and make sure the port being used is 506 and transport is set to UDP (not TCP)
  • If you have an Auth/IP whitelist section, make sure 104.237.24.84 and 104.237.20.20 are added
  • Assign the correct AI agent to 18313754500

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5

Hey @j3385mac Could you please share the relevant failing Call IDs? This will help us review the calls and investigate the issue further.

Thank You

6

where can I find the failing call ids

7

@j3385mac Dashboard — Go to Data > Call History. Open any call; the call ID (session ID) is shown in the call detail view. You can also filter by session ID.

Thank You

8

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9

@j3385mac are you facing issues on these calls

10

no but these calls are not using the sip trunk connection i configured. V2 is the live version and it looks like it has 3 phone number/connection options

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11

Hey @j3385mac Could you please share this agent id and Org/Workspace ID: Go to Settings → Workspace → General. Your Workspace ID will be listed under Workspace ID.

You should be making calls so we can verify which calls are not using the SIP trunk. Please share the relevant Call IDs for those calls as well, so we can review them.

12

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13

do you need anything else

14

All calls in Call History are still showing as coming through the forwarded Retell phone number (+18312733254). I do not see any calls arriving via the “InterContinental SIP” connection. Does that indicate that no SIP INVITEs are reaching Retell, or is there another way I should be testing the SIP trunk?

15

@j3385mac Checking it with the team.

17

Shah just checking in did we find out anything yet

18

Hey @j3385mac on our side the number is imported correctly and the last three calls all completed and transferred successfully, so the Retell account/agent config is fine. The reason your PBX engineer sees no response from sip.retellai.com is a direction/architecture mismatch, not a firewall or config issue on our end.

  • **Retell never opens a SIP session toward your PBX and does not accept probes.** sip.retellai.com only responds to a real inbound INVITE for a specific DID — trying to “connect” without sending an INVITE will look like silence. That’s expected, not a failure.
  • **Inbound (Calling Cloud → Retell):** your PBX must send INVITE sip:+18313754500@sip.retellai.com (E.164 with the leading +, host exactly sip.retellai.com). Supported transports are UDP/TCP on 5060 and TLS on 5061.
  • **Termination URI is outbound-only.** c8313754500.callingcloud2.net is used only when Retell dials your PBX (transfer/outbound leg). It does not pull inbound traffic. It’s currently saved with transport TCP on our side — if your PBX only listens on UDP for that host, let us know and we’ll flip it.
  • **The IPs you allowlisted (104.237.24.84, 104.237.20.20) look like Calling Cloud’s IPs, not Retell’s.** For our outbound/transfer INVITEs to reach Calling Cloud, please allowlist Retell’s SBC egress range 18.98.16.120/30 plus the additional ranges listed at Connect Retell voice agents to your telephony provider on Calling Cloud’s SBC/firewall. You do not need to add your PBX’s IPs on our side — Retell routes inbound BYOC by the DID in the Request-URI.
  • **Current live path:** the 3 call IDs you shared came in via a Twilio forwarding leg (from +18312733254 → Retell), which is why they succeeded but “aren’t using the SIP trunk you configured.” Once Calling Cloud starts sending INVITEs directly to sip:+18313754500@sip.retellai.com, those calls will show up here on the BYOC path and you can retire the Twilio forwarding.

Please let us know if you need further assistance.