Fireflybob - correct. Prefixed of course with the "Hold A1" bit. Or "Hold A1, standby..." if you're too busy.
Or of course, why not throw CAP413 out of the window and say "READY TO COPY" which sounds much cooler.
Apparently.
I've just done 2 sectors and heard half a dozen aircraft start a conversation with "Rhubarb 123 STANDING BY for descent."
Why do they not REQUEST descent? Is this some form of misguided hint-dropping to controllers that they should know everybody's optimum top of descent point and to waken up?