When the caller calls your number, as Steven Wright suggested, simply have your message be a busy signal!
Thank you for calling the Peoria Weather Line. (Insert appropriate weather report for the season here.)
Please hang up now if you would want to speak to Johan. Otherwise, please stay on the line to leave him a message.
(Fairly boring message:) This is John. I can't come to the phone right now, but if you leave a message at the tone, I'll return your call. (Now, re-record the message every morning. Frequent callers will notice that something sounds different, but will be confused since the words are exactly the same.)