Depends in what you call good quality.For the voice service by itself, you have diferent “codecs” that could work from 5.3 kb, 6.0 kb,9.6, up to 32 kb.
That is the voice only, After that you have the overhead, that could be up to two or three extra kilobits, depending on the equipment and codex.
You do not get a proporcional gain in quality when you use more bandwith.
With the new codecs, you will be hard presses to hear the diference betwen a 6kb compressed channel and a regular PSTN call.
The rule of thumb is that you need around 12-15 kb per call that is active, plus minus 2 to 5 kb.
Is is not a exact science like the old TDM, where you use the bandwith if you are talking or just transporting silence.