Why can't I change the default dialing app on my Nexus 5?

[Darin Hodgson]

Why can't I change the default dialing app on my Nexus 5

I have a Nexus 5 running Android 5.0.1. I installed TrueDialer as an alternative for the standard Google Dialer that comes pre-installed. I'd like to make TrueDialer the default for all calls. Based on other posts on this forum I tried accessing Settings->Apps->All and selected Google Phone. Unfortunately, the Clear Defaults button is grayed out. I also tried disabling the Google Phone but when I attempt to make a call from contacts I still get the standard Google Phone app instead of TrueDialer.