"- cut off service to all passengers except government agents (!)
All seems a bit OTT to me, especially the last - not aware of anything in the GSM system that allows it to identify a phone belonging to a 'government agent'."
Current GSM phones, and its the phones, can read the access control class from the SIM. Defined classes are:
Joe public,
Security services,
utilities (gas water etc),
Emergency services,
and phone company staff.
(See GSM spec 02.11)
On command from from the network the phone will simply not attempt to place a call for certain classes.
In the UK chief constables can instruct the mobile phone companies to disable mobile coverage for Joe Public over all or part of their forces area.