What is the ruling on using unknown-source phone credit, and what should someone who receives such credit do?

This money belongs to an unknown owner. If it is possible to identify the owner by contacting the telecommunications company, then it should be returned to him, as it is a trust in your possession. If it is not possible to return it, then it can be used, provided that its value is given in charity on behalf of its owner for general Muslim welfare projects or to the poor and needy.