I have installed Zoiper on my PC with Windows 10. Everything worked 100% and about after the 3rd call no one could hear me anymore and I could not hear anybody else. Is there something wrong with the sound card driver or something else? Is this a common problem on Zoiper?
You could be experiencing NAT related issue, however you can first try to lower the "Registration expiry time" from 3600 seconds, to 120 for example. You can do so from your account configuration menu.