Have a customer that needs to listen to sales calls from his phone, i know you can make make lines so you can pick up and listen but anyone can, need it so only he can pick up and listen in. I did it a long time ago but forgot how i did it.
system is a MICS

The quick and easy way is for the sales person to hit Feature 83 then it lets someone push the line button showing on their phone to party line listen in (and talk).

Other than that, if you have newer software, you can get a Norstar Tech to come in and set up hunt groups then you, as a supervisor, monitor the calls in that hunt group.

thank you