curlycord is right. The Do Not Disturb feature prevents all calls from ringing, including internal calls and transferred calls. It might not be what you want.
If you have no experience in Norstar programming, the first thing you should do is get a copy of the Installer Guide for your system. A PDF copy can be easily found online. Go straight to the Programming chapter and read on. If you're not sure of what you would have to do, be sure to follow curlycord's advice: ask the system manager or call in someone that can train you to make such changes.
To stop the incoming lines from ringing your phone, you need to log into programming. The settings are under Terminals & Sets > [Your Set] > Line Access > Line Assignment. Once you get there, press the SCAN softkey to go the first line that is assigned to your phone. If the line is set to "Appear & Ring", you want to change it to "Appear Only". If the line is set to "Ring Only", you want to change it to "Unassigned". If the line is already set to "Appear Only", you don't want to change it. Press the SCAN softkey again to move on to the next line assigned to your phone. These changes ensure that the buttons on your phone don't change.
As for the doorphone, there are many possibilities. It depends on whether the doorphone is a Norstar doorphone or a third-party doorphone. It also depends on how the doorphone is configured. Can you give us some clues? For example, is there an indicator flashing on your phone when it rings? If so, what is the button next to this indicator?