Hi
Difficult to give a concise answer, but you could try using the command "compvisu lio" to find the number of DSPs used for voice, and have a look at the VOIP basics poster here
http://packetlife.net/library/cheat-sheets/ to see how one can calculate bandwidth for G.711 with 20 ms framing.
But some calls could be G.723/G.729/G.722 and others things can also complicate. (I don't think you can see bandwidth used for signaling, for example)