Hi Sam
To diagnose missing of incoming audio the first step is to check if you hear "touchtones" or "dialtone" when you dial - these are local sounds and will provide a good hint about how local audio playback device is set. Next, try a call to 999 (voicemail) - it's only you and the PBX. If isn't working, if PBX is located on internet, be sure you set it into "out-of office" fields, to ensure STUN resolution of RTP ports. If PBX is on intranet, be sure you set it into "in-office" field, to ensure STUN resolution is NOT invoked for RTP ports. If you suspect a firewall issue a good idea is to to use the tunnel. If none of them works, another possible issue might be the audio codec - not probable in case of 3CXPhone and 3CX PBX, however, if you have access to the PBX, try to configure on extension's page "PBX delivers audio" - this will force PBX to transcode sound.
Regards
vali