Did you get this fixed?
What happens when you tap Phone -> Voicemail? Do you see Visual Voicemail, or does it say it isn't working? Usually the first step is to reboot the phone by holding HOME and SLEEP until an Apple logo appears. If you don't see visual voicemail, or if it hangs, go to Settings/General/Reset - Reset network settings. You will not lose any data, but will have to re-enter WiFi passwords.
Or, what worked for me, I put another sim card in my iphone from another iphone and powered it on, went to phone app and clicked voicemail, then powered off and put my sim card back in, and it worked.