bell button

noun2

1. •
   A small, round mechanical device which, when pressed, rings a bell to announce one's presence or call a servant, assistant, etc.

   • He pressed the bell-button set in the frame of a door marked 611. The door was opened immediately by a diminutive girl […] — 1931
2. •
   A button in the form of a jingle bell, used as a fastener or decoration on a piece of clothing.

   • I haven’t the advantage of being a girl with a brother and a baker’s dozen of beaux in bell buttons and gray. — 1891