We used to have the 20 minutes call rule as well for a very very long time. However it vanished when cabin duties made it impossible for the cabin staff to do just that, especially on the many very short sectors we fly.
Nevertheless the cabin crew usually has a pretty good idea on the time they have for their service and check before and after with the cockpit if everything's allright, we're on time and if we need some more coffee or meals. Most of them don't bother to call and just enter the FD which is still allowed over here (dunno if it is possible in the US).