You are on the right track!

Whats now?

The app download has started. A few more moments and off we go

You can follow the instructions below to enable the necessary settings for the installation process.

The attached screenshots are compatible with most of the new Samsung models but there may be slight changes in different devices.

1
Click on open
2
Click on
settings
3
Click on allow from this source and then click on the back button
4
Click on install
5
Open  CallApp
6
Enable the accessibility services
7
Disable auto updates
8
In order to update CallApp in the future
If this message appears, please go to the “app management” screen in the settings of your Android device and uninstall the “CallApp Contact” app before the update.
If this message appears, please go to your phone settings, choose ‘app’s management’, and then go to CallApp, and click on 3 dots in the top and then click on ‘allow restricted settings’.
Then you will have the option to turn on the CallApp accessibility. You can see detailed guide here 

Accessibility Toolbar