@tyil :blobcatlaugh: ok, imagine Phonelines, but not for homes and not for mobile phones. A Company usually have a bunch of phone lines, e.g. 10 or so for idk 100 employer... But everyone inside the company has their own phone, so you need a system that handles anything and its protocol is called SIP. Nowdays this System can be in the Internet or next to some plain old Phonelines