How to set Google dialer as the default dialer in Sony Xperia Z3+?
You may need to clear out the default for the Sony Dialer first.

Settings &gt; Apps &gt; All &gt; [insert Sony dialer app here] &gt; Clear Defaults


I find that dialing a number from your contacts may not prompt you for a new default, either. Instead, navigate to a website that has a properly formatted number like(###) ###-#### that you can tap on to have it open in a dialer - that should prompt you for a default selection. 

After your default is selected, you can disable the old dialer so it doesn't show up in your app list.