If you are having one way audio try replacing your router. I have had that problem with a few different clients and it was almost always the router (blocking the RTP stream in one direction.)

Someone please correct me if i'm wrong but,

This is caused by the router not passing through the incomming RTP stream. If the router is not VoIP aware (almost all of them in the last ages are) it won't know to pass the stream through as it comes in on a different port.

The otherway is to set you VoIP unit to DMZ and see if that fixes it. If it does then find out and forward the RTP (media ports) to your VoIP device.

There may also be a NAT keep alive option on your VoIP unit that might help.

Good Luck.