Help please with PC to PC voice calls

What AM I doing wrong please?????
I can make a video call with my contacts without any trouble at all BUT I
cannot make a voice call. All that happens---no matter if ringing or
answering a call--is that we get "You missed a call from****" after clicking
"Accept" the call. I am unbelieveably frustrated. It is just so maddening.
Please help.
Norman