Did my husband cheated on me?

We've been married for 8 yrs. now. We have a 3 yr. old. Last year, I was able to intercept a phone call from his female co-worker. She wanted a favor for him to get a food-run for her. I confronted my husband & said, she was just a co-worker. I checked his cell phone bill and cought several phonecalls he made to her and her to him. Although the minutes were not long and not alarming but the longest was 20 minutes conversation. What's up with that? I asked him about all the calls and of course he won't admit anything. He said, it was all about work. But how come he DID NOT tell me about the girl? He kept it and did not tell me that he calls a female co-worker. When I found out, I called the gilr and asked, she said my husband calls her once in a while but it was just about work, but the girl did not sound very convincing to me. She was not even apologetic about it and seemed she did not care. I got so upset and confronted myhusband. Of course, they will not admit on anything. I want to know if the phone calls are enough evidence that he cheated on me or not. My mind keeps telling me that he cheated, my intuition tells me he did. What do I need to do? After what happened to us, I have lost trust in him and I don't think I will ever trust him again. I'm still with him bec. of our son but, I really want out of the marriage bec. he betrayed my trust. It's hard for me to forget and I don't think I will ever forget what he has done. They started talking last year from Feb-May. I want to know if he cheated on me or not. I hate the fact that I'm with a person who cheated on me just because I don't have a real evidence. Is there anyway I could find out if he really did cheat on me? I am a true believer of once a cheater, always a cheater. Any suggestions are appreciated. Thanks :)