I have installed the free version of Zoiper 3.9.32144 on my Windows 7 PC. I am connected to a Mitel 3300 PBX. I can initiate a call and everything works as expected. When I receive a call I hear it ringing, but when I answer it I can't hear the other party and they can't hear me.
I've tried changing a few settings, but nothing changes. I've had other softphones on this computer and they all run just fine. I really like Zoiper though and would like to keep using it.