This button is not active if you are using Phone Manager Lite.
If you want to logon with a different extension number from that of the physical extension, click on Login>>. Check Login/Logout and select the number of the Base Extension at which you want to log on. When you start Phone Manager, you will be logged on at the Base Extension but with the number associated with your User Name. When you close Phone Manager you will be logged off and the phone will revert to its Base Extension number.