Hello
Maybe your voice sounded different on the phone, maybe it was a badly concieved joke on his part...
Did you speak on the phone further after that?
Just hypothetically saying that he thought it was someone else, I wouldn't imagine he would say 'Who is this?" would it be more likely if he did think it was someone else he would have just chatted normally until the horror kicked in that it wasn't who he thought it was originally?!
Without having the benefit of hearing how it was said, it appears he may have been shocked to hear it; do you usually surprise him like this?