Please make sure that Zoiper is allowed to use the microphone on your device.
Open iPhone's "Settings"->"Privacy".
If the microphone is enabled, but still there is one-way audio, or no audio at all, please try to disable or enable STUN according to its current state. "STUN" is located in Zoiper -> "Settings"->"Accounts"->your account->"Network Settings".
If using Zoiper with WiFi, please make sure that the routing device is not blocking ports used by Zoiper. The default ports used by Zoiper are:
SIP port is 5060
IAX port is 4569 UDP
RTP port is 8000 and above UDP
Default STUN vallues:
Server hostname /IP :stun.zoiper.com
Port : 3478 UDP/TCP
Refresh period : 30