I have a similar problem that started about a week ago. When I dial from my iPhone 6, using Zoiper Premium 1.12.1, I can hear the other side speaking, but they don't hear me. I have checked the Privacy settings on the iPhone and Zoiper has acces to the microphone. I have enabled and disabled STUN and Rport, but unfortunately this does not resolve the problem. The problem only exists on outgoing calls. When I receive a call, the sound works fine both ways!