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.