Now that you have been given the information you need to find this 'girlfriend', what is your plan. You have said that her phone goes to message instead of being answered.
Your problem is NOT the GIRLFRIEND. You problem is with your husband. He is the one who took on a girlfriend. It's entirely possible that he never told her that he is married. The man is a liar and a cheat.
I wouldn't even bother to argue with him about the girlfriend. I would tell him that I know about her and he now has two choices, give up the girlfriend or get the heck out of your house. I would only allow him to take his clothes and any personal belongings that were his before you married him.
Even if you confront him, what would you gain? Some satisfaction in venting your feelings and maybe he is ready to return to you. Until next time. If he cheats once, he will likely cheat again. You need to decide what result you are looking for and then figure out the best way to gain that result.
I'm sorry to have to tell you that your marriage is over and has been for awhile. How the situation works out is not in your hands alone. I imagine that he will deny her at first. As you add information that you have found, he will know that you know what has been going on and that you will not tolerate sharing him with someone else. Force him to decide - give up the girlfriend and pay more attention to his wife than he has been. If he refuses to choose, then you get to choose his future.