The first thing to do is alter STUN and rport in your account configuration. Try different settings with both and make some test calls. With Zoiper 3 there is another setting to try - "Open random port above 32000" for RTP. You can find it Preferences-Advanced-Network.
You need to also check your routing/firewall device for SIP ALG and disable it. Youi can also check if ports are not blocked. For the RTP it could be from 32000-65000 if "Open random port above 32000" is enabled, otherwise it would be 8000.