Longman Dictionary of Contemporary English
Wiktionary
n. 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.
Wikipedia
A dial tone is a telephony signal sent by a telephone exchange or private branch exchange (PBX) to a terminating device, such as a telephone, when an off-hook condition is detected. It indicates that the exchange is working and is ready to initiate a telephone call. The tone stops when the first dialed digit is recognized. If no digits are forthcoming, the permanent signal procedure is invoked, often eliciting a special information tone and an intercept message.
Dial Tone is code name of two fictional characters from the G.I. Joe: A Real American Hero toyline, comic books and cartoon series.
The first is male and debuted in 1986. He is the G.I. Joe Team's communications expert. He is voiced by Hank Garrett in the Sunbow/Marvel G.I. Joe cartoon series.
The second is female, and makes her first appearance in the new IDW comic series. A version also appears in the G.I. Joe: Resolute animated series.
Usage examples of "dial tone".
She checked the phone, but wasn't surprised when she didn't hear a dial tone.
It banged hollowly and for a moment nothing happenedoh, Jesus, all for nothingbut then the dial tone came.
I keep pressing some of the numbers down, hoping that when I lift the receiver up to my ear I'll be greeted by a dial tone.
He heard a dial tone, then the tone of the numbers being automatically dialed in rapid succession.