What is the ruling on someone who receives phone credit by mistake and refuses to return it to its owner, and what is the optimal Islamic way to deal with this person?

The balance that was received in error must be returned to its owner, if possible, for it constitutes wrongfully consuming people's wealth, which is forbidden. Whoever refuses to return it, while knowing its owner, has disobeyed Allah and transgressed the rights of His servants, and will be questioned about it on the Day of Resurrection. This person should be advised and reminded of Allah and the Day of Judgment. If that does not help, the balance can be retrieved through the telecommunications company, if possible. Otherwise, the wronged person should be patient and seek their reward from Allah. As for cursing and insulting, that is another forbidden act, and it is better not to respond in kind.