1.
In case you are experiencing no audio or one way audio issue, please make sure that Zoiper is allowed to use the microphone on your device.

Open iPhone's "Settings"->"Privacy" and allow Zoiper to use the microphone.

2.
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 -> VirtualGlobalPhone account -> Network Settings.

3.
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