If the audience starts by looking at the phone/laptop, you have to basically buzzfeed people into paying attention ("Oh, here's a cat picture haha, now back to our MRR discussion") for enough time to get the 30 seconds of attention to get them. Because if not, people's attention is already captured by the device
If the policy is like "don't use your laptops for the first couple of minutes", then at least you timebox the important stuff and give the talker a chance.
That's a great idea and reminded me of an SAP SME we used to have who would insert meme photos at arbitrary points of a slide-deck. Worked well for breaking the zombie-trance of attendees.
Another rule I saw was to allow people to bring laptops but leave them on a table at the back of the room. Still available for reference and emergencies but the attendees had to make a conscious and obvious to go to the laptop.
If you will actually give people information that is important to their job, they will put a laptop or phone aside and listen to you very closely. But if they're always distracted by something else, could you be wasting their time with this meeting?