Hi lads...
the IMEI number is the equivalent of the chassis number of the phone. for those in network control, it uniquely identifies the handset and is a basic element required for correct billing.
'Cloning' the phone is done at least by copying the IMEI number and a few other things.
The handset box should have a little piece of paper with PUK,PUK2 and IMEI for the handset.
If you change handsets, the SIM is newly linked to the new IMEI on their computers.