All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Riba (interest and/or usury) takes place by two parties exchanging a currency with the same currency without abiding by the Islamic conditions of exchange. As regards selling mobile phone call credit, it has nothing to do with Riba because in reality the person who buys the call credit, buys a service to call phones at fixed rates where he pays a given amount of Dirhams or Riyals or Dinars per minute, so it is permissible for the company to sell a call credit valued at 100 Dirhams, for example, for 150 Dirhams or more, and this is not called a loan; rather, it is buying and selling because when the company gives the call credit to the customer, it adds its value to his account and the fixed debt which he is liable to pay becomes the price of that credit, and this is considered as selling benefit or service (for a fixed price) and not selling money for money. What should be taken into account (in contracts) is its reality and meanings not its words and constructions.
Allaah Knows best.