First : Do you have z3, z3c, z3dual, scorpion or scorpion_windy?
Either way, you should get CyanogenMod and OpenGApps, then install them using ADB. This will allow you to have the default phone app that you want.
CyanogenMod : download.cyanogenmod.org/?device=z3
OpenGApps : opengapps.org
ADB : developer.android.com/studio/index.html#downloads
I know this isn't the place for personal opinions, but here's mine : Refund that and get back your Nexus 5. Honestly.