BAD HARDWARE WEEK

The IMEI number is used by a GSM network to identify valid devices and therefore can be used for stopping a stolen phone from accessing that network. For example, if a mobile phone is stolen, the owner can call his or her network provider and instruct them to "blacklist" the phone using its IMEI number. This renders the phone useless on that network and sometimes other networks too, whether or not the phone's SIM is changed.
However, many network and security features are enabled by knowing the current device being used by a subscriber.
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99% BAD HARDWARE WEEK: New York Attorney General wants answers on US telcos' kill switch refusal.However, if you are tracked like hundred of other millions and you want to change your SIM,  you will be disabled by this attorneys request to telcos ! YOU MUST BE TRACKED, you like it or not !
It also serves as an essential means of gathering METADATA and acquiring new targets in certain location. 
SG.09 Interception Requirements is GSMA Security Group. Closed not public, so what is inside SG.09, WE DON'T KNOW.
EXCEPT Public Attorneys and GSMA Security Group members. Telco's should be members.