BB Curve :: 8520 Stopped Making And Receiving Call

ive had my blackberry 8520 a while now when i 1st got it all the setting blackberry and phone where working then after a few days stoped making and recieving call and same with text every thing else works such as email,instant messaging,internet i am on pay as you go so when my booster ran out i topped up again while the booster takes a couple of hours to come back on the phone service worked again and i could call out and recieve calls the same with text when the booter came back on the phone service not long work and ive been stuck tryin to get help the phone shops dntno they say contact blackberry.