At a conference, people are always going to be using technology. They are always going to be checking their phones, and they are always going to have a thousand things going on. That's just the fact. They're there for a whole day, or even multiple days, emails need to be checked, sometimes coding needs to be done, sometimes they just want to check up on what the talks in other tracks are about and consider switching.
Therefore it is my job as a presenter to make sure nobody is checking their phone. If I am interesting enough, laptops start closing, phones stop being checked, sometimes they even go into pockets. As soon as I lose the audience, all that tech comes back out.
And I think that's what a lot of professors forget - BE MORE INTERESTING. Have a better presentation style. Take public speaking classes. Do something to make yourself stand out above the distraction of internet and devices.
Because here's the thing, those professors who [have to] make attendance mandatory, are without exception the worst presenters. Same goes for banning tech. If you ban it, you're just saying "I suck at public speaking and I would rather you deal with that problem than I"
And yes, it is an amazing feeling when you start with a room of a hundred people each looking at a different screen and end with a room of a hundred people looking straight at you with all laptops closed and phones/tablets put away. It's got nothing to do with tech and everything to do with winning your audience over.