Prepaid mobile phones has one basic principle you pay-as-you go meaning if you do not top up (refil your phones credit) you can not make any calls from your phone so there has to be always credit on your phone. The alternative phone billing method to this would be post paid where one makes calls and waits for the bills/invoice, it kind of has a contract (mutual agreement between service provider and the customer). The post-paid billing system is sometimes decieving because users sometimes are tempted to make longer phone calls which at the end of the day face huge bills.
It is therefore recomended for people who can afford large phone bills.
Prepaid mobile phones is a the commonly used in markets where vast majority of customers can not afford large phone bills. For futhur information on prepaid mobile phones one could check out this site.
