Answering a phone call while out to dinner with your girlfriend is the height of rudeness. Only on-call workers taking calls from work - or a call from the babysitter is important enough to answer. Also it's not your place to tell him when he should and shouldn't answer his phone. While you didn't 'demand and dictate' to know who called, you certainly insisted on knowing - which amounts to pretty much the same thing.
All that being said....his behaviour is fishy.
He didn't tell you that she called because he didn't want you to get upset. Whether or not he's got something to hide remains to be seen. Do you think that they are only casual friends now? Or does he still allow her to be intrusive in your relationship?
The biggest problem however is lack of trust. I think you don't trust him with her. You say that he's a good and trustworthy person, but if this is the case, why is he still taking calls from a girl who's got a thing for him? And if it IS just a casual friendship, then he doesn't trust you enough to be honest with you. He's scared of your reaction.
How long have you been with this guy for? Are you prepared to have this issue continue indefinitely? Because I can't see him being firm with her - he obviously values her too much to risk alienating her.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.