The data being transmitted from the BTS to the Phone and back is
enciphered. This is to prevent 'snoopers' from capturing the ID. In
the days of AMPS phones, it was aquite easy to acquire IMEI numbers
right off the air, clone them, and start making calls which are
charged to the original owner.  This lead to authentication, and in
GSM, the ID details from the SIM, not being transmitted over the air.

It also prevents people from simplyh listening to cell phone
converations, as could easily be done with AMPS phones.

The ciphering is nothing to get overly excited about. I figure the NSA
could break the codes in a second or two, but it will certain
discourage the average listener from getting anywhere!