Automatic Call Back (ACB) is a feature of telecommunications equipment for automatically interrupting an incoming connection and then setting up a connection
in the opposite direction. The Automatic Call Back feature
isavailable for modems and routers. The called modem terminates the connection
and then automatically re-establishes the connection to the calling modem. The ACB function is a security function to prevent unauthorized connections; on the other hand, it serves to charge the first called connection with the costs of data transmission.