It doesn't have to be a bell. Heck I taught Loki to press the Staples Easy Button. You are just teaching targeting
So it's not so much the sound of the bell to them as "if I ring this Mommy will let me out" I don't recommend hanging bells on the doorknob - I've used hanging bells but we hung them next to the door. They shouldn't associate bell=the door opening, they should associate "i ring bell"="someone lets me out" Does that make sense?