Competitions hosted within a community usually aren’t so effective.
Competitions rewarding individuals for individual contributions go against the nature of what you’re trying to do.
The better option, by far, is to have a competition against another community for a prize. This subtly changes the dynamic of the competition from opt-in to opt out.
In practice, you have to make no effort to not be involved in a competition to win an iPhone. It’s different when you’re in a group that’s in a competition with another community. The group you have self-identified with is under threat. You’re in the competition by default now. If you don’t contribute, you’re helping the opposition.
It’s a subtle but effective difference. Try giving another community the opportunity to win the prize. It isn’t the winning that counts, it really is getting as many people as possible to take part.