Hi everyone — I’m hitting a wall with a custom SIP trunk setup and could really use some help from anyone who’s dealt with this. Setup Phone number imported from Telnyx sip_outbound_trunk_config configured with: termination_uri = sip-eu-west.telnyx.com auth_username + auth_password set Telnyx OVP enabled for Western Europe (Level 2 verified) All transfer_call tools point to landlines in the same country (E.164 format) Issue Every warm transfer_call fails with: SIP status: 478 — Unresolvable destination twirp error: unexpected status from INVITE Logs also show: custom sip headers undefined What I’ve verified so far Inbound calls through the same Telnyx trunk work perfectly Outbound calls from Telnyx WebRTC dialer (same credentials) work → Telnyx is not the issue Failed transfers do NOT show in Telnyx CDRs → INVITE never leaves Retell Debug vars show: lk-real-ip: 18.98.16.122 lk-transport: tcp → traffic is coming from LiveKit internal SIP gateway, not my trunk Hypothesis transfer_call is being routed through Retell’s default (LiveKit) carrier instead of my configured SIP trunk. That carrier likely doesn’t support the destination country → causing the 478. I also found thread #572 which seems to confirm this, but the suggested workaround (Retell-managed numbers) isn’t viable for my target country. Questions Is there a way to force transfer_call to use sip_outbound_trunk_config? The docs mention X-Telnyx-Username header — where do you actually configure custom SIP headers? I don’t see this in UI or API Is there any way to enable additional countries on Retell’s default outbound carrier? If this is a limitation, what’s the recommended workaround? External bridge (e.g. jambonz / FreeSWITCH)? Any insight would be hugely appreciated 🙏