I need some serious advice on my relationship of 5 years.?

Okay me and my boyfriend have been together for almost 5 years and have been living together for 1 year. By the way he is 30 and I am 25. Neither of us have been married or had kids but we have been talking about getting married soon and having kids afterwards. He has never lied to me (that I know of) and I feel that we speak very open and honest about things. Well just recently my phone broke so he purchased me a phone and added me to his account. I looked through his calls just being nosey and I never imagined finding anything. His ex lives out of state and so therefore I recognized her calls. They have been in contact 3-6 times a month for the past 2.5 years. Talking for hours sometimes. I confronted him and at first he was pissed and said it was wrong for me to go through his personal things. We talked and he is now apologizing and scared I am going to leave. I am not leaving I just feel betrayed. I am not one to yell and scream I am more of the type of when I am silent you better worry. I just dont know what to do. His says they are just really good friends and he did not want to tell me cause he didn't want to hurt me but I believe he should have been honest. How can I get past this, and am I even wrong? Any good books on this topic?