Hi again everyone, We are running 3CX PhoneSystem 12.5. Repeated runs of the "firewall checker" have revealed that all ports are forwarded "properly", according to the PBX. Presence is functioning on remote extension using 3CXPhone. Remote extensions, including desk phones located in remote offices, as well as iOS 3CXPhone and Android 3CXPhone clients are able to: - place calls out over PSTN and VoIP trunks, including to remote extension destinations - receive calls placed into the PBX from PSTN and VoIP trunks, including to remote extension destinations - place calls to non-remote extensions - receive calls from non-remote extensions They are not able to: - place calls directly amongst themselves by dialing only the extension The behaviour I see is that the call is placed, and connects without any problem, but there is no audio in either direction. What next steps should I be taking to troubleshoot this problem? Thanks again for the expertise.