What is Zip tone?

In telephony, a zip tone is a call-progress tone which indicates a new incoming call is either connecting or waiting depending on the application. Unlike a ringtone, which alerts those near a telephone to answer it, a zip tone alerts someone already on the line—for example a telephone operator, call center agent, or telephone subscriber with call waiting service—that action is needed for an incoming call such as pressing a button or reciting a phrase.