This happens. It’s tricky. Mostly I ignore it. If it’s a person I know well enough, I’ll just tease them in a lighthearted way (“Hey Texty Texterson, whatchya doing on your phone?” said with a smile encourages people to smile back and re-engage with the conversation) or if our relationship is really strong I might go so far as to ask if they’d be willing to try my experiment for a day—but I wouldn’t ask them when they were ignoring me, at another time.