I've umpired for several years, you're right. That hand motion is undoubtedly an indicator given that he has the rest of the gear on.
Most look something like this and people who haven't seen them before can get an idea of how they would fit in the hand:
https://www.amazon.com/Easton-A162621-Umpire-Indicator/dp/B002N34FT4
That's definitely not a phone.
Can confirm
Source: Found this on Amazon