ssh into your phone
go to /System/Library/Carrier Bundles/ATT_US.bundle/carrier.plist
download that file to your desktop
go here Convert Apple Property list from binary to text XML and convert the file
it will save it to your computer
open the file and scroll to the bottom and you will see...


<key>apn</key>
<string>acds.voicemail</string>


is there anything in for the password and username?
<key>password</key>
<string></string>
<key>username</key>
<string></string>


there are a few of us having problems with visual voicemail i think its due to the username and password being left out