One-way audio happens when a call is setup, the called party answers and the resultant audio stream is only setup in one direction. A call may begin with one-way audio or may start with two directions of audio and then drop one side (usually inbound audio). This can easily be confused with call drop.
Check to make sure that the ALG or SPI feature in your firewall or router is "OFF". This issue is almost always (99%) an issue with the local firewall/router blocking the inbound and/or outbound stream(s) of audio.
Make sure your Internet provider is not blocking a large range of ports between 10000 and 64000 UDP. Most business-class Internet connections will not block these ports.
Questions to answer:
- When does it happen? Is it repeatable? Can you make it happen all the time?
- Always good to see if it coincides with other events on the network.
- Does it happen to everyone at your company?
- Does your Internet provider block ports? Get a list if they do.
- Who is not hearing audio?
- If you can't hear the caller, then it is probably the firewall or router. If it is the other way, then it is probably the firewall or Internet connection.
Keep in mind: It might not be you - other people have cell phones and VoIP services.