Iphone problem , please help.

Hey guys , i bought a new Iphone 3g yesterday and everything was ok untill i tried calling , im using the prepaid card service so whenever i try to call someone it says i don't have enough credit to call when i do , so i tried reseting all the settings from the General tab and when i did it said it needed an hour to complete and i accepted , the thing is its been reseting for more than 5 hours now , it only shows the apple logo for like 10 or 15 minutes and then it goes away and then comes back , what is the problem here and how can i fix it .thanks alot .