...IT guy has replaced two of the PCs...
Sorry for the slow reply, Ed, but it said Panasonic so I didn't even bother opening the thread since Wayne is the guru. That said, if the only thing that has changed is the workstations, then maybe we can assume that the phone system is still set up the way it was when they first set up the old workstations.
If that's the case, then we know the hardware is different, and likley some sofrware settings. I realize you've likely checked this, but what about the Windows Firewall? Have you disabled it? Also any/all antivirus.