One Way Audio
This error is infamous and can be quickly corrected simply by forwarding the common VOIP ports to your PBX.
If your service provider delivers audio over "direct media" port 5060 UDP must be forwarded to your internal PBX
Ports for RTP will also need to be forwarded, depending on which PBX you are running, these ports will vary
running almost any version of Asterisk the RTP ports are 10,000-20,000 UDP, and if you are running any version of 3CX previous to V12
forward ports 9000-9049 UDP.
If you are still getting one-way audio, it is possible that your service provider delivers audio via proxy in which case you will need to allow that URL through your firewall.