I think I have worked it out now. With Camp-on Tone Security/FAX Machine = no and the auto campon timer set to a time eg 5 secs and not left blank / empty on all the COS's of the deivces involved along the route, it will work IF, there isn't a 1st Alt or 2nd Alt set up to fwd when busy e.g. to VM.
For example, I have a DDI ext with no 1st or 2nd Alt set - it will just ring and ring if someone calls it. So if someone does make a call to it and I answer it.... then a second call comes in to it whilst I'm still on the line, I hear a short beep/tone and the display changes on the handset stating that a call is waiting.