which circumstances will a called party be supported by Cisco CME 3.0 Callback Busy Subscriber?

Under which circumstances will a called party be supported by Cisco CME 3.0 Callback Busy Subscriber?

A.
When at any endpoint.

B.
When an IP phone on a remote system reached using PSTN.

C.
When an IP Phone on the same Cisco CME system.

D.
When an IP phone on a remote system reached using VoIP.