top up

A prepaid mobile device, also known as a pay-as-you-go (PAYG), pay-as-you-talk, pay and go, go-phone, prepay, or burner phone, is a mobile device such as a phone for which credit is purchased in advance of service use. The purchased credit is used to pay for telecommunications services at the point the service is accessed or consumed. If there is no credit, then access is denied by the cellular network or Intelligent Network. 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 other non-telecommunications services paid for by advance deposit.)
The alternative billing method (and what is commonly referred to as a mobile contract) is the postpaid mobile phone, where a user enters into a long-term contract (lasting 12, 18, or 24 months) or short-term contract (also commonly referred to as a rolling contract or a 30-day contract) and billing arrangement with a mobile phone operator (mobile virtual network operator or mobile network operator).

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