The actual bandwidth required for VoIP depends on several factors. Most notably which compression algorithm you choose. G.729 uses about 19 to 20kb per call per direction (up and down). So that is about 40K per call. G.723 (6.3) uses between 13 and 15kb per call per direction, so about 30kb per call. This is based on 30% of silence.
Some units, like Quintum, can save more bandwidth when there is a quintum at both ends. You can get a copy of their bandwidth calculator on their web site that may help in this. It can be found at