1. A sound heard on a phone system when you pick up the phone and it's ready to make a call. In US telephones this consists of a 350 Hertz and a 440 Hertz sine wave (two pure tones) mixed together.
2. That ominous sound in definition 1 that indicates your girl/boy friend has hung up on you.
1."To make a call, lift the phone off the hook and place it so that the cord is facing downward near your mouth with the two sets of holes facing your head. Then listen for a dial tone. When you get a dial tone, dial the number." (instructions just in case you're Amish or haven't been in a house since 1927)
2. I got into an argument with my S.O. and got a dial tone.