Looking through these comments, it seems that Teams still needs a fair amount of maturation (and perhaps architectural tweaking) before replacing SfB as a phone system. The collab stuff may be better, but that's a tough sell if basic functions like transfer to voicemail aren't there, and other basic features have a confusing/poor UX (clicking a default mobile number initiates a Teams call).
Comments like "have to open the full Teams client to get call control back when needed" are also troubling: if you're doing significant work elsewhere in Teams and have to context switch at the application level rather than the OS or physical device level, it undermines efficiency. It's also concerning that the expectation of dialtone-level reliability is sitting on top of and at the mercy of a heavyweight, frequently updated app.
Please please please... make sure the fundamentals are solid. Background blur is cool tech, but it's a nonstarter when you can't do something as simple as transfer to voicemail.