A prepaid mobile phone (also commonly referred to as pay-as-you-go, pay-as-you-talk, pay and go, prepaid wireless, or prepay) 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. If there is no available credit then access to the requested service is denied by the mobile phone network. Users are able to top up their credit at any time using a variety of payment mechanisms. The alternative billing method (and what is commonly referred to as a mobile phone contract) is the post-paid mobile phone, where a user enters into a long-term (generally lasting 12, 18 or 24 months) or short term (also commonly referred to as a rolling contract or a 30-day contract), billing arrangement with a mobile network operator or carriage service provider (CSP). “Pay-as-you-go”, “PAYG”, and similar terms are also used for other non-telephone services paid for in the same way.