Using the feature Calling Line Identification Presentation (CLIP) it is possible for the call recipient to display the telephone number of the caller. Whilst CLIP was originally only available for use with digital ISDN telephony, this feature is now also supported and utilised in analogue telephony.
CLIP is also available with all other variants of digital telephony, for example Voice over IP (VoIP). With digital telephony, the transfer of the caller's telephone number or caller ID takes place via a separate signalling channel, which is independent of the voice transfer. In contrast, with analogue telephony special modulated audio signals are transferred to the recipient telephone as caller ID information whilst the telephone rings to signal the call. In order to display the caller ID the process must be supported by the recipient telephone.
A prerequisite for using CLIP is that the caller ID is not suppressed by the caller. If all the prerequisites for Calling Line Identification Presentation are met then the caller's ID will be shown on the telephone display of the call recipient whilst the telephone rings to signal the call. It is also possible to display the name of the caller, if the recipient telephone is equipped with its own configurable address book.
Legal regulations state that sales and advertising callers must activate the CLIP feature, so that the telephone number of the caller is always shown with these calls.