Interesting. Do you have several phone numbers (and Skype etc. numbers) for that Contact, and have you checked that all of them are correct for him? For example, one of the numbers could be his old office number, and someone else in that office dialled your number. The reason I ask is because the N900 incoming call log only shows the contact name, not which specific number he used to call you from, so the call might not have been from the number you think.