luvs, a couple of questions
do you think your relationship is not the best it can be lately? I'm just wondering if you have any reason to suspect that your husband would potentially cheat on you with someone else?
More than likely, he lied about not knowing how the number got there. But, depending on what your answer is to my first question, maybe he was just keeping the number as a little ego booster for himself, like 'heheh, I've still got it', but he never meant to act on it.
If you have no reason to suspect that he has any desire to seek relations with some other woman, then just wait and see what happens. if he was considering cheating, then you would have already scared the crap out of him by asking about that phone number. If you think that wasn't enough to stop him, then you two have alot to talk about.
Good luck.