10.25.2.6. Callback (CCBS) WP3 
- Only Callback on busy is supported. Callback on no answer is not possible.
- The system waits for a certain period of time after the destination station goes on-hook before activating the callback.
- During this time, or while the idle state is being signaled to the requesting party, the station cannot be seized by incoming calls(NDUB).
- The station can query the status of the callbacks (or a callback to a certain station).
- If callback cannot be initiated the A station must be informed.
- Callback request of stations which then became busy become inactive. As soon as the requesting party becomes idle the callback request becomes active. This information is entered in the callback queue.
- CCBS is only available for all services for DSS1 connections. For the Telephony service CCBS is always available (also for mixed connections e.g. DSS1 with system terminal).