I think your all wrong.
The double ding actually means "ready for the first cup of tea"
Which is why you occasionally hear it in the cruise as well.
Airbuses also have an automatic system fitted which gives a ding as the gear comes up and the tray comes out. I intially though "
me they don't muck about releasing the cabin crew" but it really means they have flicked the switch in the cockpit to turn the kettle on.
If you don't give the double bing as you go onto the runway you have no chance of a cup of tea before the top of climb.
(don't worry I know I will get what I am given, if I am lucky and if I help checking the seat pockets on a turnaround I might get a biscuit too)