Some flight attendants aren't up to date on being able to turn off the cell antenna, but most these days are. It's tended to be the crews out of the midwest that have seemed to be unaware. I fly mostly west coast flghts.
I've never had an issue on United, but on Southwest some will say they can only be used in "airplane mode" or with the radio turned off. Others will say they can't be turned on at all. The back of the magazine where it lists the approved devices says cell phones are ok in "airplane mode" so that's always a backup if you get confronted about it. But you can point to it and ask for clarification ahead of time and they'll look kinda silly to say that no you can't after it's stated in writing that you can.
Luckily there's a nice "off" showing in the corner so you can point to that. I've never had a problem although I've never turned it on when the attendant has said it can't be turned on at all.