Prioritisng IP traffic on Cisco

HI
I want to run an IP PBX (Digium / Asterisk) for local and
international calls. My Internet connection is via VSAT and very
expensive but I want to be able to have toll quality voice for 4
concurrent international calls. The IP PBX will reside on the LAN
and my serial connection will be to the VSAT. Do I prioritise on
the E0 or S0? I suspect S0 and if so, what would be the best way
to do this? (CBQ , WFQ, ) Also, anyone out theremwith
experience on Asterisk / cisco? thx in advance