So long as the phone company has not announced the additional amount and it is added to the card, then what has happened is the result of some mistake, and the one who has discovered this has to inform the company so that they can correct it. He has no right to benefit from the extra amount unless he asks permission and gains the company’s approval, because the wealth of others is not permissible unless it is given willingly; a mistake does not make it permissible to dispose of this extra amount.
The Prophet (peace and blessings of Allaah be upon him) said: “The wealth of a Muslim man is not permissible unless he gives it willingly.” Narrated by Ahmad (20714); classed as saheeh by al-Albaani in Saheeh al-Jaami’, no. 7662.
And he (peace and blessings of Allaah be upon him) said: “No one of you truly believes until he likes for his brother what he likes for himself.” Narrated by al-Bukhaari (13), Muslim (45).
But if the extra amount is added to the card with the approval of the company, such as if it announces that whoever buys a card for one hundred, they will put one hundred and twenty on the card, for example, then this comes under the heading of lowering prices, and the seller and renter may do that.
And Allaah knows best.