It was Angela waiting for him in his hotelroom. Jim walked in and heard Angela say: "D?", and he didn't realize it was Angela. He probably figured that Dwight can't possibly have a girlfriend waiting for him in his hotelroom, so it must be a hooker.

Yes, it was Angela. In the scene before, it showed her checking in under the name "Jane Doe" wearing sunglasses. Michael just missed her at the front desk.

never expected that from angela
the voice didnt sound much like angela tho