Is it permissible to use a mobile phone found on a beach when its owner has not been found?

The device in your brother's possession falls under the ruling of a luqatah (found item). You are obligated to ascertain its specifications and announce it for a full year in public places. If its owner comes forward, it should be handed over to him. Otherwise, it becomes permissible for your brother to benefit from it or to give it away as charity. The sale that took place before the end of the announcement period is void, and the mobile phone must be returned to your brother, and the value returned to you.