Possible Clues:

• Don't knock till you've tried it
• Bordello with signal fitted to entrance
• Ring it to announce presence of arrival in bordello
• Visitors ring it to signal to house occupant
• Target of an Avon representative
• Visitor's device
• Summons, of a sort
• It signals a new arrival, at a push
• One may show press instruction at an opening
• Favourite in nursery has nobbled Ebor ringer