Noun
English Wikipedia has an article on:dial toneWikipedia
dial tone (plural dial tones)
A telephony signal, usually in the form of a sound, used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call.
Left alone with the dial tone... excuse me, operator, why is no one listening? Melina Marchetta
The PBX does not offer dial tone to the calling extension: it only detects on-hook or off-hook status. Source: Internet
It had the older, traditional Bell System 600 Hz+120 Hz dial tone. Source: Internet
When a user picked up the telephone handset, the resulting line loop operating the user's line relay caused the exchange to connect the user's telephone to an originating sender, which returned the user a dial tone. Source: Internet