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This tutorial is for installing the Google Dialer on any Axon 7 device as the stock ZTE dialer sucks (in my opinion). This will also work for any other Android 6.0+ device but this is modified for the Axon 7.
Axon 7 or any other Android 6.0+ device
1) Enable Unknown Sources in Settings>Security. This is needed to sideload apks.
2) Download the Google Dialer apk from APKMirror here: https://www.apkmirror.com/apk/google-inc/google-phone/google-phone-7-0-140861421-nonnexus-release/google-phone-7-0-140861421-nonnexus-android-apk-download/
Do not download any other version it will not install!
Once the Dialer app has been downloaded click on the download notification to launch the installer. Click 'Install' at the bottom right.
3) Download Google Contacts from the Play Store here: https://play.google.com/store/apps/details?id=com.google.android.contacts&hl=en
4) Go to Settings>Default Apps>Dialer and set the Google dialer as the default dialer.
5) Go to Settings>Apps and tap the three dots at the top right. Click on 'Show system'.
6) Go down to the stock Dialer app and tap on it. Press disable.
7) Go down to the Google Phone app and tap on it. Grant the Phone and Contacts permissions. Turn on 'Draw over other apps' and 'Modify system settings'.
8) Go to Settings>Power Manager>Power management for apps>Phone and disable all power saving policies for it. Allow the app to autostart as well.
9) Go to Settings>About Phone and tap on build number 7 times to enable developer options.
10) Go to Settings>Advanced Options>Developer options and turn on USB debugging.
11) Download the 15 second adb installer from here: https://forum.xda-developers.com/showthread.php?t=2588979
Launch the installer and input 'Y' to everything, including to install ADB systemwide.
12) Connect your Axon 7 to your PC with the USB-C cable that came with the device.
13) Press the Windows and X buttons on your keyboard and click on 'Command prompt'.
14) Execute the following command:
adb shell pm hide com.android.incallui
This will disable the call UI of the stock dialer during a phone call.
15) Reboot your device.