Usually, when you want to set an app as default, you need to clear the previous defaults.

`Settings/Apps/Phone/Open by default/Clear defaults`

You can also `DISABLE` + `FORCE STOP` + `CLEAR DATA` the unwanted app to make it easier to change the default behaviour.

Then, normally, in `Settings/Apps/Configure apps/Phone app/`, you should be able to select your desired phone app.

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Do you have *z3*, *z3c*, *z3dual*, *scorpion* or *scorpion_windy*?

Either way, you should get [CyanogenMod](https://download.cyanogenmod.org/?device=z3) and [OpenGApps](http://opengapps.org/), then install them using [ADB](https://developer.android.com/studio/index.html#downloads). This will allow you to do way more than just changing the default phone app.

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I know this isn't the place for personal opinions, but here's mine : Refund that and get back your Nexus 5. Honestly.