Finding if an extension is in use

Wanted to begin by saying that this forum is great, have set up features on the phone system that I was not even aware of. Thank you to those that post here!

Now to my issue…
I have the latest build of FreePBX installed and have about 11 extensions configured with 4 incoming telephone lines.
The phones used are 2 Polycom SoundPoint 550 for the reception (extensions 101 and 102) and 9 Polycom SoundPoint 320.

How can we see on the other extensions whether the reception phones are in use?

Thank you.

Program buttons ofn the phone as BLF for the target extension.


Typo up above…

Should be:
Program buttons of the phone as BLF for the target extension.

Thank you for the suggestion. I was thinking the same thing. I was thinking to add the primary extension (101) under line 2 on each phone.
This might be a stupid question but what does BLF stands for?

Busy Lamp Field