tone

/təʊn/ noun [S] COMMUNICATIONS

Definition

› an electronic sound that you hear on a phone line: Please leave your message after the tone.

a busy tone › (US also the busy tone, UK also an engaged tone) an electronic sound when you call a phone number that tells you the person you are calling is already using the phone for another call: I've tried calling her several times but I keep on getting a busy tone.

→ See also dial tone, dialling tone

(Definition of tone noun from the Cambridge Business English Dictionary © Cambridge University Press)