When mobile equipment is stolen or lost, the operator or owner will typically contact the Central Equipment Identity Register (CEIR), which blacklists the device in all operator switches so that it will, in effect, become unusable, making theft of mobile equipment a useless business.
The IMEI number is not supposed to be easy to change, making the CEIR blacklisting effective. However, this is not always the case: a phone's IMEI may be easy to change with special tools and some operators may even flatly ignore the CEIR blacklist.