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What Ever Happened to MSN Messenger?
(www.techspot.com)
This is a most excellent place for technology news and articles.
Fun fact any developer working with the api can tell you, there is a clear distinction between de voip bit and the meeting/chat bit. They haven't bothered rewriting or integrating it in any way so the Skype for business backend is still very much alive.
Another fun fact: On the backend, Teams uses SharePoint to store files, and Exchange to store message. The whole M365 stack is a house of cards built on ancient tech. It's a wonder it works at all.