[dg7feq]

if you want landline-like quality usually VoIP uses the G.711 codec - which needs 80kbps up&down. That is 160kbps = 20kbyte/second = 1.2 MB/min

If you want cellphone like quality you can use the GSM codec which has roughly 15kbps up&down, 30kbps = 4kbyte/sec = 0,24MB/min

In normal VoIP clients you can set which codecs you want to use (this also depends which codecs your VoIP provider supports).

The betamax/finera brands for example support the following codecs:
G.711 (64 kbps)
G.726 (32 kbps)
G.729 (8 kbps)
G.723 (5.3 & 6.3 kbps)
GSMFR (13.2 kbps)
(calculate ~30% overhead for IP headers to the kbps rates)