This is asked some often. if you had searched before asking:
Cisco doesn't support this feature (call waiting annucation to caller), and has not announced plans to do.
Cisco will tell you that the other chocie is to received a busy singnal, (thing that will rpeved the calle to know about the missed call).They also will tell you that you canconfigure presence and/or espansion modules to monitor other extensions status.
I've created a TCL/IVR script that provides the funcionality, with the drawback of requiring an additional button on the the 'phone A' . This limitation does not exists on CME. It can be adquired at the website mentioned in my profile.