In telecommunication, an automatic callback is a service feature that permits a user, when encountering a busy condition, to instruct the system to retain the called and calling numbers and to establish the call when there is an available line.

Note 1: Automatic callback may be implemented in the terminal, in the switching system, or shared between them.

Note 2: Automatic callback is not the same as camp-on.

Source: from Federal Standard 1037C