When you log in to a phone, you take control of that phone. Incoming calls for you are redirected to that phone and your user information and settings are available. Any existing user on the phone is logged off when you log in.

The method for logging in depends on the current state of the phone:

Phone In Use
If the phone is already in use you can still login. The existing user is logged out.
Logged Out - Not Logged In
If the phone has no current user, it displays a basic menu with just the Login option is displayed. Note that in this state the phone can still be used to make internal and emergency calls.