Really interesting scenario here. A call comes in, the call is parked, the call is picked up. Audio both ways.
A call comes in, a user whose phone isn’t ringing picks up the call using *8, audio present both ways, parks calls, after picking up parked call outbound audio is gone. Inbound audio still works. Placing caller on hold and then picking them up off hold resumes audio traffic.
Assuming PBX is behind NAT device. Oddball one-way/zero-way audio can sometimes be blamed on a SIP ALG. If you have such an option in your router, it should be disabled. Do you have the full rtp port range being forwarded at the router to the PBX?
PBX is in the cloud with 10000-20000 UDP wide open. The customer has phones at multiple locations and phones at both locations have the same issue.