oh wow. If you really want to mess with them use this one.
http://www.payphone-directory.org/so...bell/using.wav
Also, if you want to use these it's recommended that you set your voice mail to pick up the call after 1-2 rings, the sooner the better. The reason is, if there is truly an error (number disconnected, couldn't be completed, etc) their phone won't ring, it will go straight to the error message. If the phone rings 5-6 times before going to voice mail, most people will know something is up.