I assume it's not brand-specific, but I'm not keen on gathering enough ants to test this with my Android phone.

So far, the best explanation I've seen is that the vibration basically freaks the ants out a little bit, and so they shy away from the phone (leaving pheromones telling other ants to do the same) a little bit. The cumulative effect of all the ants avoiding the phone but leaving each other "follow me" trails creates the circle. The other explanations I saw involved electromagnetic waves, which is the kind of thing people who don't understand electromagnetic waves tend to say. I'm not an ant physicist, an ant chemist, or even an ant sociologist, though, so I really have no idea either. What I do know is that this is weird. (Some are also saying the ants are CGI because they appear to pass through each other—but I think that's just because they're small and they're dark and also ants do bump into/walk over each other all the time.)

Sources: BoingBoing