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Retell AI

Hey everyone, posting here too in case it gets more eyes 🙏

I'm having an issue with Transfer Call to a SIP URI on Elastic SIP Trunking (Zadarma). Termination URI `pbx.zadarma.com`, trunk user `561227-100` — normal outbound calls to E.164 numbers work fine and authenticate correctly.

But when I use Transfer Call (cold transfer, SIP INVITE) to a SIP URI on the same domain (`sip:119@pbx.zadarma.com`) instead of an E.164 number, it always fails.

I pulled the packet capture from the call detail page and found the exact cause. Only 3 packets total:

1. Retell sends INVITE to `sip:119@pbx.zadarma.com`
2. Zadarma replies correctly with `407 Proxy Authentication Required` + standard Digest challenge
3. Retell sends ACK and gives up — it never retries with a second INVITE including `Proxy-Authorization`

So this rules out a config issue on my end or Zadarma's — the challenge is correct, but Retell's Transfer Call client just doesn't handle it when the destination is a SIP URI.

Test call_id: `call_c6a006e59b024af7f311489a177`

Anyone run into this or know if it's a known bug? Thanks!