The dial tone, also known as dial tone or trunk tone, is one of several tones used in telephone and mobile phone networks
to dial a connection. The dial tone is heard as soon as you pick up the phone but have not yet entered a call number. As soon as a caller hears the dial tone, this ensures that he is connected to the exchange, usually the local exchange, and can enter the desired call number. The dial tone is a continuous tone of 425 Hz.