I got a bit upset with a fellow regular at my local shop's casual boardgaming night. Just about every time he will have his smartphone out while we are playing (Terra Mystica was what was up that night), grinding whatever phone game he was on between turns. And often had to be told it was his turn.
When I suggested (perhaps a bit shortly) he pay more attention to the game on the table, he got short with me later, asking if we were penalizing people for taking long turns when I was trying to figure out my best move.
So playing on your phone during a game. Always rude, only okay if there is a long gap between turns, or you are out waiting for everyone else to finish the turn, or fine if you don't have to be reminded too often or take too long to get back to the non-electronic game?
When playing a game, I think we owe it to ourselves and the other players to be present and engaged. This holds particularly true at a con, where we've invested time and money to get there to play. Generally, this holds true just out of respect for others.
I'm not at my best when my focus is split, and neither is anyone else. I prefer no phones or other devices to distract myself or my fellow players.
I expect all the players at the table to prioritize the game we're playing.
I get it, smartphone games can be mesmerizing, but I'm giving my time and attention to the players at the table, and I expect everyone else to do the same.
It's one thing to use the phone to quickly check your mail, etc. Playing a full game on your phone while you're in a gaming session with other people is incredibly rude.
I always bring my smart phone and always hit one of the annoying ring tones whenever someone rolls dice. People seem to like it, especially the duck "quack" noise. It also seems to be popular during True Dungeon and at the movies.
Seriously though, having someone be on their phone playing a game...while playing a game would be annoying.
No phones at the game table.
A huge part of the reason I play boardgames is to get away from smart phones and computers and IoT devices and anything else with an internet connection (especially anything I have to write software for).
We had a boardgame night at my place with a 'no phones' rule and when I went outside to accept the delivery of tacos (yay!), I accidentally got locked out. It would have been nice to be able to call the guys and ask them to let me back in and would have saved about 20 minutes of me trying to hit the window with a rock.
For some book-heavy systems it is more convenient to travel with all the rules electronically on our smart device rather than lug around 30 pounds worth of books. I agree that playing something else on your phone is disrespectful but sometimes the phone is your rules reference and that should be allowed.