At the moment, it is not possible to change the default calling app on iPhone.
If you want to change the default calling app on Windows, first search for “Default apps”.
Then click on “Choose default apps by protocol” at the bottom.
Finally, scroll to “TEL”, click on it, and select your app of choice.
For more detailed instructions for Windows, you can go here.
If you want to change the default calling app on your Mac, open FaceTime. Under the top FaceTime menu, select Settings. In the Default for calls dropdown, select Teams.