As much as I hate to use this answer, it depends.
On a LAN, using G.711, figure on about 80 Kbps. Across a WAN, if you're using G.729, it will depend on the transport you are using. Figure on about 30 Kbps (the real number is more like 27, but it's easier to multiply by 30 in your head, at least for me) per call if you're using frame relay or point-to-point T1s. It's up around 40 if you're using ATM due to the increased overhead.
Of course, if you change the default payload from 20 ms, it will change all the numbers. It will go up for 10 ms and down for 30 ms, but so will the apparent quality of the call.
