I would say it depends on the cabin crew.

For example: Once flying into Kuwait on BA, just seconds before even the landing gear came down the guy on the window seat (I was in the centre aisle, he has no-one next to him) was whispering in his phone. I caught the eye of the flight attendent and just gave a subtle point to the guy and did the international sign-language for "phone" (fingers to ear). Well, this girl JUMPED from her seat and firmly shouted - as only a BA flight attendant can - "turn that phone off IMMEDIATELY".

On flights with fairly useless crew (I usually fly Thai between KWI/DXB and Bangkok) I just lean over or turn round - depending where the offender is - look menacing (it's a gift) and say clearly PHONE ... OFF. And it works, but this is with Arabs who don't wish to prolong the humiliation.

In general though, judging by the lack of action by MOST crews, clearly they don't think there's a danger of using phones on board. So I think they should either forget the whole phones-off announcements and let air rage ensure, OR if it IS a hazard, enforce what they're saying over the loudspeakers.