Starting from Android version 5.0 (Lollipop) default behavior of Phone icon tap was changed to open resents/favorites list. Dialpad add's another shortcut to your phone that opens the stock dial-pad directly.

Tested and working on:
Android 5.0
Android 5.1
Android 6.0
Android 7.0
Android 7.1
Android 8.0
Android 8.1
Android 9.0