How to set Google dialer as the default dialer in Sony Xperia Z3+?

I hope you are well. I have recently switched from a Google Nexus 5 to a Sony Xperia Z3+. However, I'm slightly annoyed at all of the default apps that Sony bundle with it. I have already replaced the Xperia keyboard with the one from Google, and that was fairly easy. I am now trying to replace the default dialer with the awesome Google dialer that I am used to. I managed to find online an APK for the Google dialer (all verified as genuine) and installed it to my phone. However, I cannot find the option to select it as the default dialer. I went to Settings → Apps → All → Google Dialer. The 'Launch by default' option is greyed out. Does anyone know how to get this working as I really love being able to search for a number by typing the name of a company and don't like the Sony dialer?