Here's your solution - tell her you're going to break up with her. When she asks why, tell her it's because of the phone thing, it makes you uncomfortable, you suspect her of something, and you think it will lead to an unhealthy relationship down the line. If it's nothing, and you are just being paranoid (and if she has any common sense), she will let you see the phone. If she doesn't, and is willing to BREAK UP WITH YOU over it, you have your answer.