Replace the question mark with a number

Blonde, "I'm on the road a lot, and my clients are

complaining that they can never reach me."

Psychiatrist, "Don't you have a phone in your car?"

Blonde, "That was a little too expensive, so I did the next

best thing. I put a mailbox in my car."

Psychiatrist, "Uh ... How's that working?"

Blonde, "Actually, I haven't gotten any letters yet."

Psychiatrist, "And why do you think that is?"

Blonde, "I figured it's because when I'm driving around, my

zip code keeps

changing."