Full context (rest of the ticket that got split above): the 401 retry is answered by a 500 Server Internal Error — same behavior with a BYO SIP trunk credential. Our US Twilio number on the same assistant works end-to-end, so assistant/voice/webhook are healthy — this is purely the German inbound SIP path. Two support tickets sent to support@vapi.ai (Apr 8, then Apr 13 as an URGENT follow-up explicitly flagging the 5-day silence) — both received only the boilerplate auto-reply pointing back to this Discord. Zero human response in 8 days on a paying account. Root question for any engineer: where on Vapi's side do we set the inbound Digest credentials that 3CX must respond with against realm=sip.vapi.ai ? The docs cover outbound BYO auth but not the inbound direction to a Vapi-managed SIP URI — 3CX currently sends an empty username because we don't know what Vapi expects, hence the 500. Happy to DM Call-IDs + full SIP traces. Thanks 🙏