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I'm not sure if anyone else had or is having this problem where visual voicemail doesn't work after installing the tether "hack" (found here) but I had it on my 3G and 3G S and my sister's boyfriend had it as well.

If you install the tethering hack and visual voicemail stops working, this may help. It's what I did on 3 different phones and it worked for all of them, and it's the only thing that worked.

Try actually tethering once (make sure you see the blue tethering bar at the top) and then turn tethering off. Go back into the Phone app and click voicemail. It should call your voicemail again. Hang up and exit the Phone app. Just mess around on your home screen for a few seconds and the visual voicemail password thing should pop up asking you for your password. After you put in the right one, VVM should be working.

Again, I'm not exactly sure if this is what fixes it, but that's what I did on 2 different iPhone 3G's and on one 3G S and it worked every time.