Hi, we just purchased the premium version with the g729 codec and everything works fine in our LAN but when we change into 4g data or externa WLAN we can register and complete the calls but no audio present in both ways.
You can find some instructions here:
http://www.zoiper.com/en/support/answer/for/ios/19/SIP_-_No_audio_or_one_way_audio
Keep also in mind that some mobile operators restrict the use of VoIP in their networks. For more details, check with your mobile vendor.