Presumably they want to take advantage of other functionality in the native dialer, and faking out the whole dialer interface will probably produce a huge number of other bugs. It's not unreasonable to integrate with native functions and in most cases we complain about apps that don't do that.
> in most cases we complain about apps that don't do that.
In this case I don't think that thinking applies. As I understand those APIs are there so that you can create replacements for the stock dialer app, which is not what Teams is. If that's how it's been designed then I believe it is trying to take on too much responsibility.
Why would they have to do that? All they need to do is show a keypad and capture the microphone and speakers just like any voice chat application does. Why does any dialer API have to be involved for that?