Add a line to monitor extension

How do I add an extension that I can monitor? For example, I need to see if this users is on the phone

On your IP phone? you should set BLF keys on your phone.

in end point manager?

Saw your post elsewhere about analog phones - is this the same customer?

As @psdk suggests - BLF keys on the phone. Otherwise, iSymphony would show you IP phone status, but if it’s analogue phones then I have no idea if it will work.