URGENT – Telnyx inbound SIP to Retell returns 478 recv_refuse for Italian numbers

Support Help
1

**Hi Retell team,

I’m having a **blocking issue with inbound SIP from Telnyx to Retell** for Italian customers.

**Setup**

  • Provider: Telnyx Elastic SIP trunk → `sip.retell.ai:5060` (TCP)
  • Numbers affected: **+3909711780032** and **+390302041456**
  • Each number is imported in Retell and assigned to a different agent
  • Outbound calls Retell → Telnyx work fine

**Problem**

  • All inbound calls from Telnyx to these numbers fail immediately
  • Calls never appear in Retell call history
  • Telnyx CDRs show:
    • `Sip response code: 478`
    • `Hangup Details: recv_refuse`
    • `Cause: unavailable`
  • Telnyx support confirmed that:
    • INVITEs are successfully sent to `sip.retell.ai:5060`
    • The 478 response is generated by the **Retell SIP server**, not by Telnyx

Example Telnyx `Sip call-id` values (all failing):

  • `937c0095-a6c7-4ed6-abdd-6c1494ff1b92`
  • `01f4ed4d-94e9-4c46-9e59-f39e7dec3366`
  • `29ce2185-3570-4d27-8993-32385ec766fb`
  • `ce623ce3-8d8a-433b-a218-7bf0d5776be8`

**Impact (urgent)**

This is currently **blocking me from onboarding new paying customers in Italy**.
With Italian DIDs I cannot simply forward to a US number, so I have **no workaround** without working inbound SIP.

**Request**

Could you please:

  1. Treat this as **high priority** and check if there is any **account-level issue with inbound SIP routing from Telnyx** for my organization.
  2. Verify provisioning and routing on your side for **+3909711780032** and **+390302041456**.
  3. Tell me what you expect for:
    • correct SIP URI / domain from Telnyx
    • caller ID / From format
    • any required auth, registration, or IP whitelisting
  4. Provide either a **fix** or a **temporary workaround specifically for Italian Telnyx DIDs** that I can use today.

Telnyx has already escalated this and is preparing full SIP traces (INVITE + 478 response).
I can share those here as soon as I receive them.

Thank you very much in advance.**

2

Hi @insegne.video

You’re using the wrong SIP server address. You mentioned sending INVITEs to sip.retell.ai:5060, but the correct Retell SIP server FQDN is sip.retellai.com (no dot between “retell” and “ai”). This is likely why you’re getting a 478 response.

You can see the Telnyx ( Telnyx - Retell AI ) Guide here

Thank you

3

quick update: the Telnyx → Retell inbound issue is now resolved.

Root cause: on my side I was using the wrong SIP server FQDN in Telnyx (sip.retell.ai:5060 instead of the correct sip.retellai.com:5060).
After correcting the FQDN, inbound calls to my Italian Telnyx numbers (+3909711780032 and +390302041456) are now reaching the agents as expected.

Thank you for your support and for pointing me to the correct SIP server domain.

Best regards,