Maggotts, I've been covertly filmed & uploaded to youtube! Not my finest teaching hour, I'll admit. The student concerned was disciplined because he'd broken school rules by using a phone in a lesson, & I made a mental note not to do that weird thing with my hands that I apparently do ALL THE TIME when I'm cross...
To be quite honest, there's far more embarrassing footage of me doing the rounds legitimately - a colleague filmed me making a complete bugger's muddle of a pre-planned peer assessment, & gleefully showed it at a training day as a 'how not to'. Thanks for that!
The safeguarding issue that pinkgrapefruitjuice raises re: use of 'smart' phones is something I would be genuinely concerned by, but tbh my experience is that cyber-bullying is far less prominent than it was, say, 5 years ago - it's no longer such a novel way to bait someone, & there's much more redress in that it's a recognised form of bullying. I'm not suggesting for a minute that it's not going on - but there seems to be much more awareness, among students, of how to deal with online bullies.
Phones aren't necessarily the problem anyway - LA firewalls tend to be pretty ineffective. Any of my tutor group, on any given day, could've told you the latest (ie. not yet noted & blocked) proxy URLs to use to access banned sites from school computors.
So even if you caused every smuggled mobile in a school to spontaneously combust - you wouldn't come anywhere close to putting a stop to inappropriate internet usage.
Far better to have sensible rules about the use of mobiles: ie. not taken out of bag in a lesson, unless your teacher has made a specific exception, & no using networking sites in a malicious manner. The vast majority of kids can see the sense in both of these, & will accept confiscation &/or other sanctions as the natural consequence of taking the piss.
Apart from anything else, given the tendency of technology to become cheaper over time, we're never going to win the war, however many skirmishes we might claim petty victories in.