I don't take any credit from this. I just saw it in the MIUI Additions Thread, although it had all the other stupid Sprint junk, so I just removed the apks for it and just kept the Visual Voicemail.
I tried using the Google Voice app, but it couldn't verify my number for some reason, so thats why I went with this. I like Sprints Visual Voicemail anyways.
So just boot into recovery, flash zip from sd card and reboot and you're all set.
Sprint Visual Voicemail app.