One-way audio, even when it’s going both ways, is a NAT problem.
First, make sure all of your NAT settings are correct. Pay special attention to your Advanced SIP settings. Also, double check the addresses you have listed for the “external” and “internal” addresses and make sure they are all in the server correctly.
Second, make sure that you are forwarding UDP port 10000-20000 to the PBX from the Firewall. This should help you with your Inbound Calling.