All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If the phone in question is of considerable value, then your brother has no right to dispose of it before one year elapses from the time of making an announcement about it by the means by which he thinks it is sufficient for its owner to be informed about it if he has a need for it and he wants to trace it where it is.
What you mentioned that there are many people in the city where you live, does not make it permissible to dispose of the mobile phone before the passing of this period of time that is Islamically determined.
Zayd ibn Khaalid Al-Juhani narrated: 'The Prophet was asked about finding gold, silver or bronze on the street and he replied: “Give its description and characteristics and make an announcement about it for one year…” [Al-Bukhari and Muslim]
Allah knows best.