Samsung Galaxy SIII running Android 7.1.2, the microphone doesn't work at all in Zoiper 2.17.8. The microphone does work on the device as I can record audio.
I've tried Zoiper on two other devices, one also 7.1.2 and one Android 11.0, both work fine on the same network, so it's not a firewall issue. I tried both SIP and IAX protocols, neither work.
Edit: I also tried changing the audio driver from OpenSL to "External Java Driver", no effect.