بسم الله الرحمن الرحيم
If the company allows your brother that anyone can use his mobile with his permission then you may use your brother’s mobile with his permission. And if the company has conditioned that it shall pay your brother’s mobile bill that in case he (your brother) should use the mobile then you may not use your brother’s mobile even with his permission as you will put the burden of the bill on the company unlawfully.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.