Without more information about your network, it's all conjecture, but 99% of all One-way audio problems are NAT related. Either you have something set to NAT that shouldn't be (a phone, the PBX, etc.) or you need to configure your NAT correctly. You also need to make sure that your router is redirecting UDP traffic to the PBX (RTP traffic uses SYN packets to set up the audio, and you may not be providing the correct forward routing path to the PBX).
Those are the common guesses.