with the same AI assistant, Twilio will tranfer perfectly but with Telnyx and Zadarma, at the point of transfer, it just hangs upt he phone call immediately at the transfer moment. Please advise how to fix or what to do next. thank you.

Zadarma specifically needs the SIP verb "PREFER" which our platform does not support. Please try using the SIP verbs "DIAL" or "BYE" for the transfer tool if you are using Zadarma. I have a feeling that Telnyx is similarly configured, especially if the call is ending upon transfer.