Whilst being called every 20 minutes is a bit extreme (and indeed a nuisance and distraction on some of our super-short sectors), I think this captain may have been wound up by previous experiences with some crews. In my experience there is a significant but growing minority of cabin crew who pay no attention to this particular rule and excuse themselves with the "Well if they want something they'll call us" line. The rule is not about getting a nice cup of tea, its about making sure both the guys are still functioning normally. If I'm dying of thirst then I'll call you, but on some flights I've noted the lengthy period during which there's been no indication from the flight crew that we were still alive and still nobody has bothered to check, even when the flight wasn't busy. Some crews are excellent and check at suitable intervals, even on busy flights, but others really just can't be arsed and its they who need a good kick up the backside before they decide they can't be bothered to comply with any of the other safety rules.