Quanta banda consuma il voip?

Dipende principalmente dal codec usato, qui sotto ne elenco alcuni:

GSM - 13 Kbps (full rate), 20ms frame size
iLBC - 15Kbps,20ms frame size: 13.3 Kbps, 30ms frame size
G.711 - 64 Kbps, sample-based Also known as alaw/ulaw
G.722 - 48/56/64 Kbps
G.726 - 16/24/32/40 Kbps
G.729 - 8 Kbps, 10ms frame size