I think it means visual voicemail is not available (but that you still have an unheard message) - this is what it looks like when you have it unlocked on a carrier like T-Mobile. What happens when you go to voicemail?
i have the same prob but i have 3.0 beta 5 i think its just att new 3g conversion the signal goes in and out at least once a day when i know i should have signal