Actually you are right to a degree, but found there is no need to go to this extreme.
Simply disconnect mobile data and hey presto those voice calls work much better.
It's often caused by the phone desperately trying to use 3g, even when the signal is congested (bars make no difference)
We can see this in action in out office all the time. in the day, it flaps between edge and 3g and 2g.
At night, same office, rock solid (and fast-ish) 3g.
So you may get the "signal", but using it is a waste of time.