When we do testing in schools to determine giftedness it is the top 95th percentile of different tests. It wasn’t just reading and math but also nonverbal tasks (like tangram type things). We used state testing and IQ scores as well. We tried to create a whole profile of a child and then determine which ones met the criteria of requiring gifted services (95th percentile and above). I don’t think there’s a federal guideline so each state (or even each district) sets their own parameters. The twice exceptional kids were the ones with ADHD or other diagnoses. But yes, it was possible that these kids were not the “smart, model student” though I’ve had plenty of those as well.
The issue is kids aren’t making phone calls in class. They’re on social media. They’re listening to music. It’s usually not obvious and disruptive to others, but still impacting their ability to focus and learn. Banning them outright causes kids to be sneaky and resentful, but allowing them can be detrimental, especially with the impacts on mental health. If you collect them before class (I’ve seen teachers use shoe holders hanging on the door) kids will bring in an old phone and turn in that. If you use the locked bags, same deal. If you scramble the service so they phones are unusable then you can run into issues in a real emergency.