Skype made the call negotiation go through a central server (as does all systems nowadays). Skype was originally built on Kazaa technology to punch through firewalls without a central coordinator and that’s what Microsoft removed. They didn’t remove it to track the calling but to enable larger group calls on weaker devices which required video mixing on a central system rather than peer to peer call (where weaker peers couldn’t decode that many video streams). Calls up to 4 are still routed peer to peer if the backend can find routes through all firewalls.

Very very little of Skype was in the new Teams if anything. Teams was a rewrap of Communicator calling tech and was a response to Slack. The real time chatting had nothing to do with Skype either.

Skype lingered in Microsoft for a couple of reasons; Microsoft was crap at acquiring businesses back then, thinking that a hands off approach was best. It meant Skype never really became a proper Microsoft team - they still felt and acted like Skype employees and they didn’t manage to affect Redmond very well. Being acquired is super hard especially when almost all of the bigger business was in a different time zone and a different culture.

I was at a leadership development workshop with a tonne of Skype leaders about 10 years ago. They were still feeling incredibly frustrated and not understanding what was expected of them. It was a botched acquisition and the fault was on both sides.