# Stuck on SIP 478 "Unresolvable destination" for BYOC outbound **URL:** \ **Category:** Support Help\ **Tags:** integrations\ **Created:** [May 15, 2026, 2:00pm UTC](https://community.retellai.com/t/stuck-on-sip-478-unresolvable-destination-for-byoc-outbound/2725 "2026-05-15T14:00:13Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![hello9](https://yyz2.discourse-cdn.com/flex008/user_avatar/community.retellai.com/hello9/32/690_2.png) [@hello9](https://community.retellai.com/u/hello9)\ **Post date:** [May 15, 2026, 2:00pm UTC](https://community.retellai.com/t/stuck-on-sip-478-unresolvable-destination-for-byoc-outbound/2725/1 "2026-05-15T14:00:13Z")
Hoping someone here has run into this. I’ve been at it five hours over two days and I’m out of ideas on the customer side. We’ve got a BYOC setup — Twilio Elastic SIP Trunk, AU local DID +61485069111, inbound bound to a Retell agent that’s been happily running in production for a couple of weeks. Today I tried to wire the same number for outbound on a new agent and every single dial fails with the same error: “Error dialing to user, SIP status code: 478 SIP error category: unknown Error: twirp error unknown: unexpected status from INVITE response: sip status: 478: Unresolvable destination (478/TM)” Three attempts, identical error every time: call\_4e34cf2bb8fccd946637dc9aea8 (yesterday) call\_6a19cc12a3ee9fad27c1b18448c (today, mid-debug) call\_05a846c8cc9a050649e047d0902 (today, latest — after I bound an outbound\_agent\_id to the number) org\_Jj4FW8q6nppFsADK if anyone from the team is looking. The weird part: Twilio Support has fully verified their side. Termination URI on the trunk ([spoke-voice-au.pstn.twilio.com](http://spoke-voice-au.pstn.twilio.com)) is configured, the IP ACL has all three of your documented BYOC source ranges (18.98.16.120/30, 143.223.88.0/21, 161.115.160.0/19), the number’s bound to the trunk’s Numbers tab, AU Geo Permissions are on for both Low-Risk and High-Risk for Toll Fraud, no sub-accounts in play. And critically — Twilio’s Debugger shows zero error events for any of my attempts. Meaning the INVITEs aren’t even reaching Twilio’s app layer, despite all the IP whitelisting and DNS resolving cleanly. So somewhere between Retell’s SBC sending the INVITE and Twilio receiving it, something is failing — but Twilio sees nothing and Retell’s log just says “478 Unresolvable destination” without telling me what destination it couldn’t resolve. What I’ve tried on the Retell side (in case any of this is the issue): - Setting the outbound trunk config termination\_uri to every format I could think of — “spoke-voice-au”, “[spoke-voice-au.pstn.twilio.com](http://spoke-voice-au.pstn.twilio.com)”, “sip:spoke-voice-au.pstn.twilio.com”, with port, without port. Retell always normalises it back to just “spoke-voice-au” so I can’t tell what it actually sends on the wire. - Transport is locked to TCP via the API (only thing it’ll accept). - Auth username/password is blank — relying on IP ACL. - outbound\_agent\_id was unset originally, so I bound the outbound agent to the phone number today. No change. - Agent is published at version 1 with a webhook URL set. Inbound binding on V20 is verified untouched. The Route header in the call response shows sip:18.98.16.122;lr which is in your documented /30 block — that’s a hint at where you’re trying to route the response back to. But it’s not telling me what Request-URI you’re actually sending the INVITE to. What I think someone here might know: 1. When Retell’s SBC throws SIP 478, what destination is it actually trying to resolve? Is it the Twilio Termination URI, or is the SBC constructing something else upstream? 2. Is there an org/workspace/account-level switch to enable outbound BYOC that I might have missed? Inbound on this same number works perfectly. 3. Has anyone here actually got BYOC outbound working from an AU local DID via Twilio? Want to confirm the combo is supported and not running into some regional restriction. Happy to run any diagnostic from my side. SIP trace, packet capture from Twilio’s side, whatever’s useful. Just need a hint on which thread to pull next. Thanks! ---
**Author:** ![Shah-Fazal](https://yyz2.discourse-cdn.com/flex008/user_avatar/community.retellai.com/shah-fazal/32/2522_2.png) [@Shah-Fazal](https://community.retellai.com/u/Shah-Fazal)\ **Post date:** [May 15, 2026, 2:46pm UTC](https://community.retellai.com/t/stuck-on-sip-478-unresolvable-destination-for-byoc-outbound/2725/2 "2026-05-15T14:46:51Z")
Hey @hello9 I’ve escalated this issue to our team for further investigation. We’ll keep you updated as soon as we have more information. Best regards ---
**Author:** ![Shah-Fazal](https://yyz2.discourse-cdn.com/flex008/user_avatar/community.retellai.com/shah-fazal/32/2522_2.png) [@Shah-Fazal](https://community.retellai.com/u/Shah-Fazal)\ **Post date:** [May 15, 2026, 3:51pm UTC](https://community.retellai.com/t/stuck-on-sip-478-unresolvable-destination-for-byoc-outbound/2725/3 "2026-05-15T15:51:01Z")
Hey @hello9 It looks like the last few calls were successful. Could you please try again and provide a recent call ID from today?