what are the functions of phone drectory?

A phone directory consist of white pages, yellow pages, there are some other color on pages but it depends on the country. White pages has postal or street address of the person subscribed in that particular area. And the yellow pages is the list of business establishment and government establishments as well. Though now a days many of us use mobile phones and give less important to land line phone and don't give much attention to phone directory. But still phone directory is important, especially to the person whose stranger in a particular place, it is listed in the phone directory the important number to be dial in case of emergency.