Hard to Reach

A blonde is speaking to her psychiatrist. “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 figure it’s because when I’m driving around, my zip code keeps changing.”