Bismihi Ta'ala

It is very encouraging MashaAllah to see your regret after performing such an act. Rasoolullah Sallallahu alaihi wasallah says, " A believer doesn't steal in the state of Imaan". This means that at that moment, Imaan leaves his body. This is very dangerous.

However, now that you regret your actions and wish to compensate for it, the following points must be adhered to:
1. Giving in charity the amount you gained from the sale of the stolen phone.
2. The same phone, or if not a brand new one, either the same model or something equivalent in value must be given to the person whom you stole off. He doesn't have to be told, but you must leave it on his desk or somewhere he could access it, with a note so he is aware of why he is receiving the phone.

And Allah knows best.