well voip is voice over ip. that means u use a definate ip i.e. internet protocol to connet to a different phone. Now when u connect to GPRS with your phone u actually connect to a router of the service provider but its not always that u connect to the same router, every time to disconnect and again connect GPRS u connect to a different router.
So, you dont have a static ip i.e. ur ip address always changes.
for VOIP u need to have a static ip.
so through GPRS its difficult to avail VOIP service of ur own.

u can use push2talk application that is a VOIP service but provided by ur GPRS provider.

for ur own VOIP service u need to have a static ip provided by ur service provider.