1.go to http://www.icopybot.com/download.htm
2.download and install ibackupbot for your os
3.make a back up of your iphone in itunes
4.open ibackupbot
5.select your backup
6.edit these two files
SystemConfiguration/preferences.plist
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replace phone to att.mvno you will find two "phone" in this .plist replace both to att.mvno
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Library/Preferences/com.apple.mms_override.plist
use this text:
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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>MMS</key>
<dict>
<key>GroupModeEnabled</key>
<false/>
<key>MMSC</key>
<string>http://mmsc.cingular.com</string>
<key>MaxMessageSize</key>
<integer>1048576</integer>
<key>MaxRecipients</key>
<integer>10</integer>
<key>MaxVideoBitrate</key>
<integer>131072</integer>
<key>Proxy</key>
<string>66.209.11.33:80</string>
<key>UAProf</key>
<string>http://www.apple.com/mms/uaprof.rdf</string>
<key>UserAgent</key>
<string>SonyEricssonW800i/R1L</string>
</dict>
</dict>
</plist>
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6. Then click the checkmark on those two file and restore to the iphone using iBackupBot software
7. It should restore and after reboot MMS and data should start working.
Note that you may need to follow these instructions again if MMS stops working after an iOS update.