How can I use/upload/record a prerecorded (mp3) voicemail greeting? I can not find a solution. I feel like this should be simple, but no. I tried playing the mp3 file on my laptop and running a 1/8" to 1/8" cable from the headphone jack to my iPhone. That doesn't work. The iPhone still records from the built-in mic. Maybe I need a special cable? Is there, perhaps, an app that will allow me to play the mp3 on the phone itself while simultaneously recording a voicemail greeting? Ideas?