[Arnt]

An android app isn't modal in that way. There is no "user mode". There are permissions instead, and the closest thing to "user mode" would be an app that has the 2-3 most common permissions, and no others.

https://play.google.com/store/apps/details?id=com.microsoft.... lists what Teams does or can do, and it's a long list that includes "directly call phone numbers". That means to call phone numbers without the usual indirection through the phone app, ie. Teams can replace the phone app.

So, Teams can do that because it isn't "user mode". There are others too, https://play.google.com/store/apps/details?id=com.simplemobi... is one.