I have the same problem.
I thought that it was a problem of the FREE version so I bought the PREMIUM VERSION.
I have the last ZOIPER version Zoiper5 5.2.6 for Windows 64bit - Windows 10
Here is the complete description of the problem:
-) You receive an incoming call
-) the popup INCOMING CALL appears
-) You click on ACCEPT
-) the popup INCOMING CALL still remains
-) you start speaking but the phone still rings and the popup is still flashing
-) after some seconds the call ends
Thanks for your answers
Claudio