Thank you Ivan,
I have tried the different combinations of audio settings, but unfortunately no difference. Isn't there a list of settings that is guaranteed to work with iOS 9.2.1 in combination with Zoiper Premium 1.12.1?
The approach seems to be very much trial and error and takes lots of time (which I don't have). If I don't find a solution for this problem soon, I will be obliged to go looking for another soft phone :-(