Depends upon the codec you are using. G.711 uses about 80 kbps in each
direction, g.729 takes about 16 to 24 kpbs in each direction. So it is
easy to do the math on how much capacity you need, and what your
bandwidth budget is when you factor in traffic from other services. If
you operate in a cisco world, they have info on their site for traffic
engineering your outbound traffic. And if you have good relationship
with your upstream provider, they could use the same rules to ensure
traffic is regulated into your pipe.
Ray Burkholder