These two posts, Setting Default Dialer and Setting Nexus Custom Dialer, make it seem straight forward, but it is not.
I am not prompted to choose which dialer to use, it only uses Google Dialer
Neither the Phone nor the Google Dialer apps have defaults set, thus the clear defaults button is not enabled.
I consider myself tech savvy and have already spent half an hour trying to figure this out. Unacceptable for something that should be easy.