I don't want to poke the bear here...but you don't have access to his phone log remember? There is no way of knowing if he truly knows her number or not....is there? If you aren't seeing a detailed log, and don't have access to his phone...you simply have his word.
It's very common for a cheating spouse to lie to our faces...that's part of infidelity. I really hope he's not doing that, I'd like to give him the benefit of the doubt...but that's why it's so important he give you full access to the phone. If she was/is calling or texting him you would be able to see it in a detailed phone bill.
