The CCBS supplementary services automatize repeated efforts to reach a currently busy or unreachable subscriber.
If the caller invokes the call completion service, the network tries to reach the busy (unreachable) subscriber for a given time in the background. When the call can be finally established (i.e. the subscriber has hung up the active call or becomes reachable), the caller is notified by the network through a ringing of his phone. When the caller picks up the phone, the call will be finally established.