Definition: an electronic sound made by a telephone Level: B2
我拿起听筒，听到了熟悉的拨号音。
I picked up the receiver and heard the familiar ___tone of the dialling signal.
他试图打电话，但线路上没有声音。
He tried to make a call, but there was no ___tone on the line.
持续的忙音表示电话占线。
The continuous ___tone indicated that the number was engaged.
