If you experience one-way audio due to NAT, disabling/enabling STUN and rport could help. Both options are located in "Config"->"Accounts"->your accounts->"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 between 32000 and 65535 UDP
Default STUN vallues:
Server hostname /IP :stun.zoiper.com
Port :3478 UDP / TCP
Refresh period : 30
Additionally, try changing the Audio Driver in "Config"-"Audio" to see if that would affect the issue.