Exceptions and prepaid. The only postpaid exception is Verizon's non-iPhone Global Ready 3G Phones, but the good news is that the carrier is happy to provide an unlock code for these devices.

A prepay mobile phone (also commonly referred to as pay-as-you-go, pay-as-you-talk, pay and go, go-phone or prepaid) is a mobile phone for which credit is purchased in advance of service use. The purchased credit is used to pay for mobile phone services at the point the service is accessed or consumed.

Once you buy a pre-paid phone card, you are given a PIN (Personal Identification Number) and a toll-free access number. When you want to use your card, dial the toll-free number and then enter your PIN. The call is connected to a computer operated by or for the card issuer.