There are a couple of other things you can look into:
If your own phone's microphone is picking up too much ambient noise, and/or its gain is set too high, it could be causing the problem. Try muting the mic and see if the noise diminishes.
VoIP can detect when there is no (well, very little) sound on the call. When that happens, it can stop sending audio packets, to conserve bandwidth. This is called "Silence Suppression". You can turn this function on, which will make the call very quiet when nobody is talking. Some people find it confusing, as it may seem to your called or calling party on the other end that the call was disconnected. If you want to experiment with it, the setting is in the CODEC section. It's disabled by default (meaning, that the CODEC is encoding whatever it "hears", regardless of the audio level). When you enable it with a check mark, it will stop encoding audio below a threshold of "silence".