What is an telephone directory?

A telephone directory, also known as a phone book, is an alphabetical listing of people and their phone numbers. If you want to phone someone, and you know the name but not the number of that person, a directory can enable you to look up the number. One problem with phone directories in the 21st century is that they only list numbers for land-line phones. We do not have a cell phone directory. And cell phones are very popular. So it is getting harder to look up phone numbers.