I have this site that has an analog polycom phone they use to call places like Zoom and Webex. I just started looking into this issue and my first tests show calls to these on a Comcast sip trunk results in the user hearing beeps. I set this to dial out through the customers AT&T sip trunks and expected audio is heard. I also can place calls to these sites via a teleworker 6900 phone and hear correct audio response.
Just want to know if anyone has dealt with a similar issue so as to eliminate some testing time