CLIP (Calling Line Identification Presentation) is a supplementary GSM service used to show the number of a caller. When a call is initiated, the caller’s MSC provides the destination MSC with the caller’s identity. The destination MSC then checks to see if the phone being called has subscribed to CLIP. If it has, then the caller’s identity is presented. CLIP couples with CLIR to provide an advanced version of caller line identification in GSM networks.
The presence of CLIP - and CLI generally - can be very useful for choosing to forward or even discard calls from certain people or organisations, as it allows identification of the caller without answering.