Hello
This issue is not caused by Zoiper, it is related to the alsa system. At the moment we do not plan to implement pulseaudio support for Zoiper Classic.
We are working on the new Zoiper5 for Linux that will not have the issue. I can not provide an exact time-frame at the moment. Please send us an email to labs@zoiper.com if you would like to be notified when the first builds for Linux are available.