I don't think I would want to use a phone for a clock. That said, I doubt any would be tournament legal, though I would need to look at the regulations to be 100% certain.
I doubt that you could use a smart phone chess clock at a tournament. For casual play, maybe okay... i would prefer a real chess clock. i think in a blitz game a smart phone just might move around a little much, and might even end up on the floor. chess clocks have rubber feet for a reason.
I don't think I would want to use a phone for a clock. That said, I doubt any would be tournament legal, though I would need to look at the regulations to be 100% certain.