Should I let it bother me?

My boyfriend and I have been having problems recently, and one of the most drastic is his relationship with another girl. He never physically cheated on me, but their constant flirting and talking struck me as a little more than friendship... (she would sit on his lap, stick her hands up his shirt and tickle him, and once went so far as to call me an estranged lover) I immediately went to him and told me how much his relationship with her bothered me, and he ignored it and kept on acting the same way. I then told him again, this time threatening that I wouldn't be able to stay with him if he kept doing this, because to me, it felt very much like betrayal. Eventually, he realized how much it hurt me that he was carrying on this relationship with her and promised he would never speak to her again, something that I didn't put him up to. I was surprised, but I took it in stride. So far, he has done very well and been very honest about everything, which I really value because I have issues trusting him. Yesterday, he told me that she had called him and they had a brief conversation about something trivial. However, today, we were sitting together when he saw that he had a missed call. He checked the phone number, and when he did, I saw that he had an outgoing call to this girl. He had called her yesterday. I feel very betrayed and lied to, but according to him, she called him and he missed the phone call, so decided to call her back. That wouldn't bother me, but I don't know whether to believe him. She texted him and he ignored her, but he feels compelled to return her call? I don't know whether I should let it bother me, given my previous experience with her, or if I should just let it go. I'm just worried because he felt he had to lie to me, and it makes me wonder whether he has really been honest all this time. Especially because he recently confessed to me that, had we not been together, he would have considered her for a romantic relationship. What should I do? Should I let the lie about the phone call slide?