Greek Yellow Pages can prove its real value. Specifically, Greek Yellow Pages offers free of charge to its advertisers a unique phone number for each advertisement. This number is unique and it is shown on the advertisement, while this specific number is not been displayed at any other advertising medium. Thus, every incoming call received by the advertiser come exclusively from Greek Yellow Pages and demonstrates the responsiveness of users to advertising client. The total number of calls that comes up upon the completion of the measurement (during the 12 months that a catalogue is being circulated) translates to "Display Performance of the Customer in the Greek Yellow Pages".