VOIP use about the same ammount of bandwith on bothe direcctions. For that reason you will need symetric bandwith, or you will limited by the upload speed. You never get to use the diference (unles you do some web surfing on the same link,not recomended).
Be carefull!!! Not every sat.internet provider is good for voip.Depending on the sampling rate, what is good for data could be unusable for VOIP.
30 channels of PSTN , using 12-15 kb of bandwith each. Depends a lot on type of equipment
A terminal like Quintum, with the packet saver option could save you big bucks on bandwith.