My husband just doesn't GET why I was upset with him about this situation... do you?

yep its suspicious! i don't like the smell of this one. not one bit. if i were you i would install a nanny cam somewhere in the room of the phone and while you are at work you will be able to capture what he talks about with this bimbo. it will surely put an ease to your mind once you find out if what he is telling you is legitimate or not.