tone
noun [ S ]
COMMUNICATIONS uk/təʊn/ usan 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