why wont my nextel i860 phone make calls? help?

[jenny]

ok so i got the i860 and i put a boost mobile sim card on it, (it always worked with boost sim cards before) and now when i try to make a call it says " service not available" and on the home screen it also says "no service" why?? i already paid for my boost mobile so the plan is active so i should be able to make calls, also the signal bar is full so i don't get it