Depending of the codec that you are using:
if the codec is G.711
15 * 80 Kbps (Ful duplex)
if you are using G.729
15* 24 Kbps
if you are using g.723.1
15* 17 Kbps.
So you need to add the overhead of the protocol level 2 that you are using:
Ethernet, Frame Relay, ATM, HDLC, etc.
If you have doubts, you could contac me.
If the media is a WAN N