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Based on the packet trace, it definitely looks like the Transfer Call client isn't completing the Digest auth flow after the 407 challenge, which would explain why SIP URI transfers fail while E.164 calls succeed.

Since Retell ACKs and never sends a re-INVITE with Proxy-Authorization, this feels more like a client-side handling issue than a trunk configuration problem.

Have you tested whether the same SIP URI transfer works from another SIP client or PBX against the same Zadarma trunk to fully isolate Retell's behavior?
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