I think you have the right to be angry. It must be annoying, but you should try to deal with the situation calmly. If you get stressed out about it thigs will start to tense between you and your boyfriend and might find your self in a bad position.
I understand his motivation to answer her calls. It is hard to ignore someone who you cared about at one point in your life. Brobably you would have done this if you were in his place, think of it this way. But if he is not making an action then you dont have to worry. He is with you in the UK and she is far in the USA, so the ball is in your field and you have to know how to play well.
I blame him for one thing though, that he didnt tell you about the whole thing. But again may be he was worried that it will affect your relation. I think you have to talk with him about it and try to sort it out and move on.
Good luck and remember you have been together for 5 years it would be a shame to lose him.