A prepaid mobile device (also known as pay-as-you-go (PAYG), pay-as-you-go, pay-and-use, go-phone, or prepaid) is a mobile device, such as a phone, for which credit is obtained. purchased before using the service. The credit acquired is used to pay for telecommunications services at the time the service is accessed or consumed. If there is no credit, the cellular / smart network denies access.
Users can top up their credit at any time using a variety of payment mechanisms. (“Pay-as-you-go”, “PAYG” and similar terms are also used for services other than telecommunications paid with a deposit in advance).
The alternative billing method (and what is commonly referred to as a mobile contract) is the postpaid mobile phone, in which a user enters into a long-term contract (12, 18 or 24 months) or a short-term contract (also known as a renewable contract or 30-day contract) and billing agreement with a mobile operator (virtual mobile network operator or mobile network operator).
A prepaid cell phone provides most of the services offered by a cell phone operator. The big difference is that with prepaid phones, payment for the service is made before use. As calls and messages are made, and as data is used, deductions are made from the prepaid balance until there are no funds left (at which time the services stop working). A user can avoid service interruptions by making payments to increase the remaining balance.
Credit card, debit card  or online payment processors.
Direct withdrawal from a bank account through an ATM   
Shop at a retail store with a retail “recharge” or “recharge” card. These cards are stamped with a unique code (often under a scratch pad) that must be entered into the phone to add credit to the balance.
Shop in-store with a swipe card, where the balance is automatically credited to your phone after the retailer accepts the payment.
Retail store or online purchase: a person can recharge a prepaid phone in another country by requesting “international recharge”. Migrant workers will often send prepaid top-ups internationally as a form of support.
Other cell phones on certain networks that provide international recharge services, where the initiator of the recharge is often a migrant worker who wants to add minutes to a family member’s prepaid cell phone at home.
Directly from some open circuit prepaid cards equipped with a mobile recharge service.
By electronic recharge in which a specially designed SIM card (the retailer’s SIM card as defined in the Philippines and India) is used to recharge a mobile phone by entering the mobile number and choosing the amount to recharge. This process is widely implemented in the Philippines and India so that anyone can be a reseller of prepaid charging, creating availability of charging stations nationwide, even in remote areas.
The credit purchased for a prepaid mobile phone may have a time limit, such as 120 days from the date the last credit was added. In these cases, customers who don’t add more credit before maturity will see their remaining balance depleted when those credits expire.
There is no requirement for a prepaid mobile phone user to top up their balance. To maintain revenue, some operators have devised reward schemes designed to encourage frequent recharges. For example, an operator may offer some free SMS to use next month if a user recharges a certain amount this month.
Unlike postpaid phones, where subscribers have to terminate their contract, it is not easy for an operator to know when a prepaid subscriber has left the network. To free up network resources for new customers, an operator will periodically remove prepaid SIMs that have not been used for a while, at which point their service (and associated phone number) stops. The rules for when this removal occurs vary from operator to operator, but can typically occur after six months or a year of inactivity.
In 2003, the number of prepaid accounts grew compared to previous contract accounts, and in 2007 two-thirds of all mobile phone accounts in the world were prepaid accounts.