Depending on the game, asking 100% attention 100% of the time is not reasonable.

I've played high crunch games where people took multiple minutes per turn, and we had 6+ people at the table. That's 15 minutes between your turns, plus GM time. It's not reasonable to expect people to pay 100% attention to 15 minutes of other people calculating which square gives them the most bonuses.

People are not going to do that, phones or no. If they don't have phones, they'll doodle, or look at advancement options, or something

What is reasonable is to expect that people pay attention when they need to. In the situation above, I don't care if someone checks out for most of that time. I do expect that they pay attention as it gets close to their turn, and are fairly ready to act when their turn rolls around. If they are, then I don't care what they do. If they don't do that, then I also really don't care why they're distracted - the fact that their distraction is slowing down the game is the problem, not the particular form of the distraction.