History of prepaid mobile phones
The history of prepaid mobile phones began in the 1990s when mobile phone operators sought to expand their market reach. Up until this point, mobile phone services were exclusively offered on a postpaid basis (contract-based), which excluded individuals with poor credit ratings and minors under the age of 18 (the typical age of contractual agreements).
Source: Wikipedia — History of prepaid mobile phones (CC BY-SA 4.0)