From AT&T’s website:

Get AT&T Enhanced Push-to-Talk (EPTT) and use your phone like a walkie-talkie. EPTT lets you communicate quickly and efficiently across organizations with the push of a button and can be added to any existing line of service with voice and data plans.

• Use your phone as a walkie talkie with unlimited range.
• Communicate with one person or a group.
• Connect in an instant with the push of a button.