When you make or take a call next time, check the signal strength. Is it at a full 4 bars or is it at 1-2 bars? If its the latter, that could be the reason. Does the poor quality happen at certain locations, or all around?
Its at half bars most of the time. Isnt half bars sufficient for calls though? It seems kind of random, sometimes calls are fine, most of the time they are not.
Subjective. If you're only getting 2 bars in 3G, then you're in an area that has awful reception. I mention 3G because calls are done in 3G.