Why isn't my iphone (AT&T) voicemail working?

My visual voicemail has not worked since I got the iPhone 5 ( about a month ago) everytime I go to the voicemail it just automatically dials "voicemail" then it says please enter the mailbox number. I attempt to enter my phone number-- that doesn't work either. When people call my phone they don't even get my voicemail! What the hell?

Its best to call customer support. (i know you may know that already.sometimes we just want to fix things ourselves than to sit and wait for an operator to answer.)