yeah what network are you on..... NOKIA make the phone but a company like Orange or O2 provide the service for your calls and you top up with them. you have to have one of their sim cards in your phone and then you can top up by either, buying a voucher and registering the voucher number, using a swipe card which you have to register with your simcard or you can top up at a cash machine.
Postdog's right, it's the network you're on that determines the number you dial to top up. O2 is 4444, vodafone 2345, t-mobile 150, virgin 789, orange 450.