Give me something I can take,
Can take to make the memories fade.
Poison kiss, remember this,
I never was meant for this day.
I think you were right to look through his phone. If you have asked him a question and he won't tell you the truth or talk about your issues what else are you supposed to do to find out the truth?? I think he is cheating or has slept with her at least once. It can be hard to admit this or see it when you're actually in the relationship but the facts you have given us say yes he has. At least now you know after looking through his phone. If you hadn't who knows how long you would have continued the relationship with that horrible nagging gut feeling!
Leave him (if you're not getting sex from him you're better off alone, as sex is a big part of a relationship). Good luck and make sure you are confident and demand that men treat you well in the future!
Just dump him. His being upset about the phone is just blowing smoke. He cheated, you found out. There isn't anything more to say.