Abstrac
The year was 1994, when Telia introduced the service CLIP (Caller Line Identification Presentation) in the Swedish telephone network. This service could be used provided a caller ID is connected to the phone socket and that the user subscribes to the CLIP from Telia.
   The CLIP service sends DTMF (Dual Tone Multiple Frequency) code over the network to the phone socket during the ringing signal. This DTMF code is then decrypted in the caller ID and the number calling is shown on the display.
   Today this service has grown so large that almost every Swedish household have a caller ID. The main task for a caller ID is to show the caller’s telephone number, but today it also exist caller ID with phonebooks and possibility to block unwanted calls.   Blocking numbers is a relative new service on the market and may be bigger in the future because the people required more control over their home telephones.
   In this project we used a microprocessor to create a prototype of a working caller ID with phonebook, blocking functions, clock and possibility to log calls.

Projektrapport [PDF]