With the Completion of Call to Busy Subscriber (CCBS) feature, a connection
is only automatically established with a busy destination connection as soon as the busy status is lifted at the destination connection. However, automatic callback must first have been activated by the caller at his terminal. If this requirement is met, the called party's exchange
waits until the called party hangs up and signals this back to the calling party's exchange, which then picks up the connection. Completion of Call to Busy Subscriber is described by the ITU-T in Q.733.3. CCBS does not work on private branch exch anges and is limited to a single carrier's network. This feature cannot be used across networks.