For phone numbers in your location (United States) use

System requirements

To be able to use the application you need a phone with the Android OS version 4 at least, but the full functionality is available with Android 6 and higher (the operational system version can be found here: open the system Settings, scroll all the way down to "About phone", click on it and there you will see the "Android system version").

When opened for the first time the application will ask you to grant some system permissions. If your operation systems allows you to (this functionality is accessible with operation system 7 and higher) please also grant the usage of the application as the default phone application. Only with this settings you can use the application for 100 %.