Why do you say that DY? If he has no history of unfaithfulness, if their friends don't think he's likely to be cheating and he didn't sound like there was anything to be worried about when goodgirl called initially couldn;t he be telling the truth?
My boyfriend has done the same thing and rang my mobile one morning when I've been partying in London all night and its been answered by a guy, passed to a girl next to me and then onto me and I haven't thought to make excuses or be emabarassed because nothing happened other than 3 people falling asleep on the same bed. My boyfriend isn't being guillible he's just giving me the benefit of the doubt and he knows that if something did happen then I would tell him about it.