What should I do for Allah to forgive me for taking a phone I found and not returning it to its owner, who called me, 15 years ago?

Whoever takes a phone with the intention of possessing it and refuses to return it to its owner is considered a usurper. For repentance, one must return the right to its owner. If it is possible to reach the owner, the phone must be returned to him, and the decrease in its value due to use and the passage of time must be paid. If that is not possible, then the value of the phone must be given as charity on behalf of its owner. Its value is estimated at the highest price it reached during the period of usurpation, because any increase in the value of the usurped item belongs to its owner, and any decrease is guaranteed by the usurper.