Busy signal

In telecommunication, the term busy signal has the following meanings:

1. In telephony, an audible or visual signal that indicates that no transmission path to the called number is available. Synonym busy tone.

2 . In telephony, an audible or visual signal that indicates that the called number is occupied or otherwise unavailable. Synonym reorder tone.

Source: from Federal Standard 1037C and from MIL-STD-188

HTTP equivalents of the busy signal include 500 Internal Server Error (generic), 503 Out of Resources, and 403.9 Forbidden: Too Many Users (generated primarily by Microsoft software).