The abbreviation CCBS is for the technical term "Completion of Calls to Busy Subscriber", a telephone system feature. It can be used when a call is made, but the busy tone for the called party is heard. In this event, CCBS can be activated by performing certain actions and the caller will be called back by the telephone system as soon as the desired subscriber ends their current call. If the caller takes this new call from the system a new connection is initiated to the originally called party. If the line is still free, they can take the call and the connection is made.
The great advantage of the CCBS service feature is that the caller does not need to repeatedly make multiple calls to try to reach the called party, but as soon as their line becomes free again this is signalled directly by the telephone system. This saves time and allows the caller to continue working in the meantime. CCBS is a feature that can only be used internally within a system. It can be provided for the entire system or only for individual participants.
Call-back requests are easily activated using a short code. Not to be confused with CCBS, an automatic redialling feature, which is activated in the phone device itself. It is used when a caller gets an engaged signal and their phone then calls the recipient at a user-defined time interval until the line becomes free again.