With a prepaid plan, a cell phone owner can buy a finite amount of minutes up front as opposed to signing a contract with a monthly subscription rate. Carriers are increasingly targeting prepaid plans towards teens and consumers with poor credit in order to boost subscriber growth.

Expiration dates range from 30 to 90 days, and when the expiration date is up, customers can roll leftover minutes onto the next card they purchase.

Under most prepaid policies, once the policy expires, remaining minutes are lost and the phone number is reassigned to another user.

Some services allow you to roll over any unused old minutes, as long as you add new minutes before your old ones expire.

Many services also always allow you to call 911, even if all your airtime minutes have expired. In any event, if you 're thinking of buying a prepaid cell phone, look carefully at the fine print. Because of airtime expiration policies, you may still end up paying the equivalent of a monthly fee, even if you never "use" the time you buy.

The per-minute cost of prepay plans gets lower when the consumers buys a bigger block of time. For example, a $20 prepaid card works out to an average per-minute charge of 30 cents while a $40 prepaid card works out to an average per-minute of 25 cents

Buying a one-year card, you can cut your wireless expense to less than half its full-service cellular price. Prepaid service providers like TracFone have an annual card that gives you about 240 minutes or so and permits you to keep your service alive for entire year. After 365 days, you simply buy another card to roll over and extend the minutes you didn't use, including any other prior card time purchased.

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