The Grandstream entryphone can be used for all the following:

• allow access to callers via the front door
• call Sue in the office
• call other apartments

You should have been supplied with a small manual for this, but it doesn't tell you how to use the phone.

Quick User Guide
Tunstall user guide
Brochure
Datasheet
User Guide
Resident User Guide

Drag your finger from the top of the screen downwards and it will show some options, one of which is Screen Off

… Darkness …

until you touch the screen again.