No, you can't hack your girlfriend's phone conversation unless you want to risk a hefty fine and maybe some jail time, but you can talk to her and tell her your suspicions.
Trust in a relationship is so important, if you don't have that it can't really work. If you have reason to believe that she is cheating then sit her down, tell her calmly why you suspect that, and ask for her side of things.
Chances are, there will be a perfectly reasonable explanation but if she is cheating you can leave the relationship with your head held high as you have done nothing wrong.
Stoop to phone hacking though, and you not only commit a crime, you make yourself as bad, if not worse, than her.
Think carefully about whether you really suspect there is something going on or whether it is just jealousy because you are frightened of losing her to someone else. Don't accuse her of doing anything, just tell her your fears and say that you are feeling insecure and can she reassure you.
I am sure you will be able to tell if she is lying. I really hope she isn't and that you can work it out - good luck!