So i have tried this with both the 3CX softphone and an SIP app on my iPhone 3Gs. I can make and receive calls, however i cannot hear inbound audio. I have confirmed that they can hear me via the use of a land line telephone. Could this be a firewall port issue, or does SIP use one port for all audio, both directions? I am new to SIP and PBX so please excuse my many questions. Please respond asap, thanks in advance.