Hi, I had a related situation a few months ago.
I had suspicions the passenger beside me had his phone still switched on as we taxied down the run-way. He had appeared to be sleeping during the safety briefing, so I politely asked him had he remembered to turn his phone off. He snapped at me that he had. Mid-flight he took his phone out and started surfing. (I wasn't deliberately looking but the distinctive BBC news website came on and that is the truth, how he picked up a signal I do not know.)
I went down the aisle to the flight attendants who were serving refreshments and explained the situation. They said they would talk to the gentleman. As it happened, as they came by the 'ten minutes' to land bell sounded, so they asked him simply to turn off his device because we would be landing shortly.
Two minutes later he got out of his seat and went to speak to the flight attendants. He complained that he felt he had been singled out!!
I found this out after we landed when the flight attendant came to speak to me and told me what had happened.
Unfortunately, this isn't just an issue about following 'house safety rules', but it is also about basic courtesy and respect and unfortunately, there are a few folk around who don't have these simple qualities.
Usually if I see somebody whose phone I suspect is switched up I simply explain that I am a nervous flyer(the truth) and that I'm sure they don't want to make me even more nervous. Most folk are very polite and re-assure me that their phone is off, but there is always the exception...
My 'solution, is that where crew find somebody with their phone switched that they should confiscate the phone. The phone should be passed to ground staff on arrival and the passenger in question should be asked to follow the ground staff to the terminal, where details of the 'incident' are recorded on the customer's notes (in the event that there is a return flight pending and/or they are a frequent flyer). The passenger should then be given a written caution and told that if this situation arises again, they shall be banned from flying with the airline in question