You can try to change the audio driver for Zoiper. In order to do that, open Zoiper, go to "Config" ---> "Audio"---> "Audio driver" and select "External Java Driver". Exit Zoiper and restart your device.
I can also recommend that you try changing the audio codec from Zoiper -> Config -> your Account -> Audio codecs. Make sure that you use only recommended codecs by your VoIP service provider.