a paper about GSM. The cell tower and phone negotiate the highest level of crypto that both mutually support.
During call setup a handset sends a packet that states its crypto level and the tower will return a signal with what is
actually going to be used. Devices that intercept GSM clear-data simulate a tower and proxy the data to the real
tower. They drop the 'I support crypto' packet from the handset so the tower thinks that the handset does not support
any type of encryption and transmits the call in the clear.