[AC Question]

need to register the phone.

[Rukbat]

IMEI number.

Run Phone, as if you were making a phone call. Dial *#06#. That's your IMEI, the number that's unique to each handset. (Serial numbers can be duplicated - two manufacturers can have the same serial number on different phones. The IMEI identifies the exact handset, model, manufacturer, etc. There's only 1 phone in the world with a given IMEI.) That's the number most network operators want to register the phone.