visual voicemail issue - but can't change settings

[discoross]

Hi all,
I get the "Please dial the 10-digit mailbox number of the person you wish to dial" when ringing my own voicemail. Visual voicemail screen looks normal. ATT tells me I have voicemail but it hasn't been activated. I've come across several posts on the issue, but none of them have solved the problem yet.
I tried #00220# (I think that was it) to reset forwarding settings.
I tried Reset Network Settings.
Many posts say to go to General>Network>Cellular Data Network but this last option (Cellular Data Network) is missing.
My carrier.plist (ATT) reads:

<key>apns</key>
<array>
<dict>
<key>apn</key>
<string>phone</string>
<key>password</key>
<string></string>
<key>type-mask</key>
<integer>1</integer>
<key>username</key>
<string></string>
</dict>
<dict>
<key>apn</key>
<string>acds.voicemail</string>
<key>password</key>
<string></string>
<key>type-mask</key>
<integer>2</integer>
<key>username</key>
<string></string>
</dict>
<dict>
<key>apn</key>
<string></string>
<key>password</key>
<string></string>
<key>type-mask</key>
<integer>0</integer>
<key>username</key>
<string></string>
</dict>
</array>
<key>voicemail_context</key>
<integer>1</integer>

Any suggestions?