Automatic Callback

In telecommunication, an automatic callback is a computer telephony calling feature that permits a user, when encountering a busy condition or other condition where the called individual is unavailable, to instruct the system to retain the called number and to establish the call when there is an available line or when the called number is no longer busy. Automatic callback may be implemented in the terminal, in the telephone exchange, or shared between them. Automatic callback is not the same as camp-on.

Source: from Federal Standard 1037C