Before you read this answer, I'd have to outline that there are pretty big legal implications to hacking someone's phone.
There have been a number of high-profile cases of jail sentences being dished out to people that attempt this.
And the NewsCorp celebrity scandal is (currently) still an ongoing case, so think before you do anything silly with this information.
How to hack someone's phone
What I could suggest is accessing their voicemail using a "spoofing caller ID method". This will allow you access to all the saved voicemail messages that are stored on the device.
How do you do this? Perhaps a video might be the easiest way to show you:
As for regular text messages - I'm not 100% sure how you would gain access to those without using some sort of password detection technique.
Feel free to write an answer yourself if you discover a way!
I would also say that, if you're resorting to this to check up on your girlfriend - then the relationship might already be dead anyway.
Hope this helped :)