Is a person's act of taking a mobile phone found in the street, and discarding the SIM card inside it without knowing its owner, considered theft, and what should he do with the phone now?

The device found by this person falls under the ruling of a lost item (Luqata). The finder must announce it for a year in the place where it was found and where the owner is likely to be. If the owner comes and identifies it, it must be handed over to him. As for taking possession of it and disposing of it immediately upon finding it, this is impermissible. It is an act of aggression for which the perpetrator incurs sin, but it is not described as theft. The one who did this must repent to Allah and announce the device while guaranteeing its full value to its owner.