Okay, thanks for the update! So I'll make sure I don't use the laptop until after climb-out to start of descent (if it is essential to use it in-flight at all!)
As for the phones, I am aware that they "squawk", looking for a cell, all the time they are switched on and I make sure mine is switched off from the time I leave the gate until I arrive back in the destination airport building - but I have a feeling that the "standard announcement" could be a little inadequate in that it doesn't emphasise enough that "switched off" means just that, not "don't make calls", and I suspect that there are often phones on brief-cases in the overheads or in pockets that are switched on.
Mobile phone vendors could also make this point strongly in the instruction books, but I think the cabin announcements could be more emphatic. (BTW I was surprised to read in the owners' manual of my new car that it also suggested that mobiles should only be sed with "outside antennas" to avoid messing up the electronics in the car!)
I'll certainly continue to keep an eye out for phones in pockets that are "blinking" after the flight is boarded!
Thanks again for the clarification.