way to stay connected with their college age students and have some control over the costs. Disadvantages Sometimes, pay-as-you-go customers pay more for their calls, SMS and data than contract customers. In some cases they are limited in what they can do with their phone – calls to international or premium-rate telephone numbers may be blocked, and they may not be able to roam. These limitations are usually the results of deficiencies in the prepaid systems used by the wireless carriers, as technology has evolved to the point where these are easily managed by triggers or APIs to third-party solution providers. Current models being deployed by wireless carriers are capable of setting the price points for all services on an individual basis (via packages), such that higher pricing is a marketing decision. Churn As described above, prepaid accounts require a subscriber to have a credit balance in the account to make outgoing calls (except emergency calls, which are always allowed). In the US, incoming calls are also barred, while in some other countries, a prepaid mobile with zero balance may still receive calls and texts. This results in an "on/off" or "all or nothing" proposition for the prepaid service providers and their clients (i.e. the account either has enough credit to use the phone, or it does not). Some operators (e.g., Orange) allow their pre-paid customers to have a small negative balance to allow short calls or texts when the customer's credit has been completely used. This is then deducted when the customer next adds more credit. The time most likely for a prepaid customer to switch to a different service provider is when the prepaid account reaches a "zero credit balance". Like other service providers, mobile service providers losing a mobile account call it "churn". Roaming In the early years, a prepaid mobile phone could only be used within the network of the operator from which the customer purchased the phone. It was not possible to roam onto other GSM networks when using the phone abroad. This was because the operator had no way to bill calls in real time from another network. However, most prepaid phones now offer roaming using one of the following methods: The prepaid mobile phone user dials a "trigger" number from the foreign location using a USSD message which is not charged for while roaming. Upon receipt of the USSD, the customer's operator will