Possible Clues:

• Informal response to ''Who's there?''
• Words after "Open up"
• Phone ID?
• Doorbell ringer's response
• 'Hello ___' (Todd Rundgren hit)
• Response after knocking
• Words upon removing a disguise
• Words from a well-known caller
• Response with a ringing doorbell
• "Mulder, __": Gillian Anderson biography