You can't listen in on your girlfriends phone calls, there isn't an app for that! Reason being, you cannot access people's devices without their permission, whether it be to listen to their calls, read their messages or access their location.
There are legal implications for this, but also ethical ones.
Have you considered why it is important to you to listen to her calls? It sounds like maybe you have issues trusting her, which is probably something you should discuss with her.
It may be that she has given you reason to feel like this, or it could be your own neurosis, but either way it could cause problems in your relationship in the future, so you should try and resolve it now.
If you find that you have no reason to distrust her, you will be far more relaxed and happier, and it could be really beneficial to your relationship with your girlfriend.
