how i transfer audio from my macbook pro to my iphone 3g

Im trying to create a voicemail box for my customers to call for monthly info and I want to include a song in the message (greeting). This sounds awful just holding up the phone to the computer so I was wondering what cable exists that will allow me to go from the macbook headphone jack to the iphone headphone jack and accept audio. I tried using a standard 1/8 male and of course that didnt work.
I perused the internet for about an hour and gave up.
Thanks for any help you can provide!
macbook pro iPhone OS 3.0