AboutPresence monitoring allows you to visually monitor other lines (extensions) to know whether or not someone is on an active call. The busy lamp field (a light on the handset) indicates which line is busy or idle; allowing you to see whether or not someone is available. Presence monitoring capacity is dependent on the number of busy lamp fields on the handset model; see your phone's user manual to learn more.
Common Use Case
You know your boss has been on a call this morning, but his 9:00 appointment is here. You glance at your phone and see that he is still on his call, so rather than knocking on the door you quickly send him a chat message: “your 9:00 is here when you’re ready” and he pings back “okay, I’ll be right out—thanks”. Definitely beats knocking on the door and interrupting his call.