First please try to answer the incoming call from FXS port, Make sure the line is ok, If so then check your network or change the codec. If the voice is low , I think you can change it in the web with this value "rxgain".
I didn't meant low volume, what I tried to say is that the quality of the incoming voice is poor.
Using Asterisk CLI command "sip show channels", always use the ulow (g711) codec for incoming calls. Is there a way to force transcoding to g729 or to g722?
Sure, But it is inefficiency. By the way waht do you mean about the voice quality .Noise/echo or low voice or something else?
If has echo , Please enable the echocancel value;
If has noise, Please try to answer the call with Analog phone which connects FXS port, Not use IP phone;
If low voice,please adjust the rxgain/txgain value;
Please describle your problem .