You say SIP trunk , NBX SIP trunks ? Calling site to site ?
So a call comes into site one , transfered to site 2, and then sent to site 3 with no audio ?
Can you replicate this with normal calling by transferring outaside calls to different tels ? Sounds like something is either blocking the audio or the route to the tels via the router may be an issue after the third transwer attempt
Would prob need a trace of a good call and bad call as it sounds like something on the network possibly . When did this issue start occurring ? All audio is RTP as in SIP mode we changed the Audio from NBX to SIP ( RTP ) .
I don't think anything in set up could cause this . The network or asome sort of bug . Only a trace would show us as I have not heard this one at all .