Camp-on busy signal

In telecommunication, the term camp-on busy signal has the following meanings:

1. A signal that informs a busy telephone user that another call originator is waiting for a connection.

2. A teleprinter exchange facility signal that automatically causes a calling station to retry the call-receiver number after a given interval when the call-receiver teleprinter is occupied or the circuits are busy. Synonym speed-up tone.

Source: from Federal Standard 1037C