Airlines are required on all planes seating 20 or more to have a visual and audio interphone system installed. It allows communication between all crew members.

The lights up front next to the microphones and the audio "bells" are all part of this system.

The same "bells" are used for attendent calls, so everyone here is correct to a degree.

2 bells on taxi is for attendents to sit. Same on approach. Also pretty sure that 2 bells means flight status change (climb to cruise, cruise to approach) as well as when seat belt warnings are given, etc...,

I was looking for something to clarify on the web...couldn't come up with anything except the [link|http://www.risingup.com/fars/info/part121-319-FAR.shtml|rule requiring their use].