How to get Android to ask for my choice of phone dialer?
Go into settings  &rarr; apps. Open the menu, and select show system apps. Now scroll down to the stock phone app press on it  &rarr; set as default  &rarr; clear defaults. Now, whenever you select a number to call, it will ask which app to use.

If you don't see the clear defaults option, then go to settings  &rarr; apps. Open the menu and press on reset app preferences this will clear all your app preferences, including which app should be the default.

Don't worry, this will not delete any data.