It depends on the type of codec compression you are using for Voip and on the type of equipment you are using.
For example G723 uses some 6.3 kbps of bandwidth, and on top of this you must add any protocol overheads (such as RTP (or proprietry) and UDP headers).
Also you need to know if your internet VSAT is symetric. You might have higher bandwidth to you than reverse (asymetric). Also are you going to carry data traffic ? how much ?, how congested is your provider ?