Hawkeye uses half a dozen cameras for its ball tracking. I reckon you could get a half decent gully version working with two (ideally three) but even then you'd need to calibrate them and have some way of having them communicate with each other to give you the final ball tracking info.
The umpire holding his mobile phone in front of him isn't enough, because the camera won't be in the same spot all the time, the bowler will probably release the ball from out of frame, the bowler might obscure the path of the ball during his follow-through, there are plenty of problems.