It happened to us at the end of a LHR-HKG. In fact, we didn't know that the phone was there, but it was the only likely place left once it had gone missing during the last 10 minutes of the flight. The cabin crew (ie the CSD plus the three F cabin crew) insisted on staying with us until the engineers turned up to dismantle the seat, and whilst they did so and then improvised a method of extracting the phone after they'd located it. And walked through to immigration with us afterwards. It was all much appreciated.