Full Skype delegation abilities
With Skype - when you are added as a delegate, the phone line of the user you are managing calls for shows up on your IP phone and you are able to see when they are on a call, have answered a call, etc.
On Teams, although the delegate function is available on the app, once added, you don't see the bosses line on your phone automatically (unclear if possible at all). They have to keep me on as a delegate on Skype for this to be available. As long as it is like this, we cannot fully cutover to Teams and leave Skype.
It would be ideal to fully integrate all Skype's features relating to IP phone use with Teams. Adaptability with IP phones are a huge reason why we have the licenses we do and don't want that to be at risk when switching to Teams.