“Praise be to Allah.
The relationship between the customer and the phone company is a relationship between a seller and a buyer, and the product being sold is the phone service.
The money that the buyer pays is in return for the phone service, and does not come under the heading of buying money with money. Hence there is no stipulation that the amount paid and the credit added to the phone be equal.
So there is nothing wrong with this credit being less or more than the amount paid, because the matter is based on mutual consent between the two parties to the transaction.
What the company offers of extra credit, in addition to the usual price, may be one of two things:
Either it is by way of lowering the price, so what is worth twenty is sold by the company for ten, and so on.
Or it comes under the heading of a gift that is given to the user in order to encourage him to buy more.
There is nothing wrong with either of these two things.
And Allah knows best.”