Dialtone

In the 20th century, when automated telephone systems were first being deployed, telephone companies noticed that customers were often confused by the apparent lack of response (before this, a telephone operator would answer), and would often assume the phone was not working. To avoid this, exchange systems would play comfort noise, back then implemented as a small buzzing sound into the line. Nowadays, the typical dialtone is a constant single tone at 425 Hz. Experienced musicians will be able to tune their stringed instruments to a U.S. dial tone.