From the second hit on an InterSearchWeb -
Blacklisted, Blocked or Barred Handsets
Originally Posted by UnlockMe
Amazingly it is only recently that the altering/changing of IMEI numbers has become illegal! Home Secretary David Blunkett introduced a new law making re-programming IMEI numbers punishable by up to five years in jail.
...and if you think about it, which IMEI number are you going to change to? You will need to have one that it recognised by one of the airtime providers, and you will need one that isn't already in use otherwise you will be imitating someone else!
Just buy a PAYG from Asda for a tenner - far less hassle.
mb