We are using Retell.ai with our own phone numbers integrated via VoIP/SIP trunking from a Turkish telecom provider. Recently, when calling these numbers, the caller immediately hears a busy tone even though no one is using the line and there is no reason for it to be busy. The SIP logs on the VoIP side show repeated "INVITE (SDP)" requests followed by continuous "180 Ringing" responses, but the call never connects and eventually drops to busy. Most critically, no call records or logs appear on the Retell.ai dashboard at all, indicating that the incoming calls are not reaching Retell’s servers. We have already checked trunk registration, authentication (both IP-based and credential-based), origination/termination URIs, codecs, firewall rules, agent assignments, and ring duration settings, but the issue persists. We need help identifying where the problem lies in the SIP integration between our VoIP provider and Retell.ai.