Please explain your system flow in more detail. Original call comes in on what (GV number in question, another GV number, POTS, SIP, etc.)? Receptionist answers how, e.g. analog PBX connected to Phone ports of all the OBis, GV forwards the call, OBi forwards the call, etc? Receptionist does what (e.g. sends hookswitch to the OBi and dials *98 from the PBX)? OBi sends outbound leg on what (same GV account, another GV account, another service)? Transferred call sent to what (same GV account, another GV account)? If call does not go to voicemail and is picked up live, what is the path (picked up on another GV forwarding phone, picked up on the OBi)?
Is OBi firmware current? Can you test one unit on your "normal" Internet connection? If you transfer to a different voicemail service, e.g. a cellphone, does that work correctly?