Ribbon bar section labeled Find > Search People > Type in First or Last Name, and click on the person you need contact info of:
Outlook on the web
Search box at the top right > Type in First or Last Name, and click on the person you need contact info of:
Phone > Contacts > Groups (top left) > Exchange Global Address List: type the first 4 characters of first or last name, and tap Search.
Note you can use a space as the 4th character. Example: Tab<space> in case you don't know if my name is spelled Taber or Tabor.
Reach someone on their office extension
Option 9 is a hidden menu choice in the auto attendant. It will let you dial by last name, to reach someone with an office extension.