For those who are having problems getting Visual Voicemail (VVM) to work on thier Iphone I have two solutions that have been successful:
1. Reset your network settings through the settings>general>reset menu. resetting the network won't delete your personal information, it will just refresh the data connection between the phone and the network.
2. If the first solution does not work: Call customer service @ 1.888.211.4727 or head to a corporate sprint store and ask to have your Voicemail Password reset. The rep will take care of the rest for you
CAUTION: this may delete any old voicemails that you have listened to already and saved so if they are important be careful and try solution 1 first.
I also reccommend, if you are heading to the sprint store, to back up any information or data you do not want to lose (contacts, pictures, email,texts, etc.) before going there because they may need to perform a hard reset which will wipe EVERYTHING.
So far these have fixed any problems with VVM that have come up.
Please post and let me know if it works for you