CLIR (Calling Line Identification Restriction) controls the presentation of caller identity (via CLIP) in GSM networks. If CLIR is enabled, the caller’s MSC indicates this restriction to the destination MSC. The identity is then not forwarded to the destination mobile station.
There is a GSM override function for the CLIR that is available to organisations such as the police, and allows the caller ID to be seen even if they have elected to restrict their identity.