One way voice can have nothing to do with compression. A faulty DSP card on a certain gateway could however cause this. A couple of other possibilites are that voice packets (RTP/UDP) are being blocked in one direction by a firewall or router access list for certain calls. Another is that the speech path is not being cut through correctly in the forward direction. It could, depending on the architecture in use, even be a basic routing problem somewhere (call signalling is routed OK but RTP stream between certain GWs not so).
If you are using Cisco they have a useful article on their website for troubleshooting one-way voice.