It's coward behaviour.
How do I get my girlfreind's messages and call details?
If you're trying to download your girlfriend's message and phone calls, then I would honestly suggest you guys just break up.
If the relationship has got to this stage, then it is sincerely better for you both to call it a day. Once the trust is gone, it will never come back, no matter what you find on her phone this time - the doubt is in your head for a reason, and this has tainted the relationship.
If you just want information about how to hack phones, rather than real valuable relationship advice... Then I suggest checking out the following videos and links.
I used them to hack into my ex-girlfriend's Samsung Note 2. We are not together anymore because of what I found, and I wish I had never seen the messages and images that were on her phone. It was heartbreaking!
So, you don't trust her? If you can't trust then you might as well call it quits. Also, would you like it if she was asking that same question?