I use freepbx with 8 users in a network. Currently I use PhonerLite. This works very well. Unfortunately I can not see who is currently occupied and who is free. Is there a way to indicate who is free and who is not? Or another SIP phone in which one sees this?
You need a phone that supports BLF so you can program them to monitor the desired extensions.
There are lots of ways to do this, including old favorites like iSymphony and FOP2.
If you’re crafty, you could also write a small program that would monitor the line status of the phones through the Hints and make your own.