Wiktionary
n. (context British telephony English) An audible signal to the calling party that indicates failure to complete the requested connection; a busy signal
Usage examples of "engaged tone".
The engaged tone caused him to slam the receiver down with a force that threatened to shatter the entire telephone.
I checked my watch at 19:34 and thought that would be unfortunate, wouldn't it, if Pepperidge finally called me and heard only the engaged tone while Al was asking about his girlfriend's health or his aunt's or whoever the hell it was on the other end, my chest's easier but there's still this cough, and the doctor says I do not care what the doctor says, just get off the line.
He left the phone on the bed, engaged tone beeping mindlessly while he dressed, then took the Porsche on a long, frosty, starlit drive north, almost to the Cairngorms.