How to send MMS on iPhone 4?

Okay so I have a prepaid iPhone 4 and it's on at&t. It use to be able to send MMS up until Apple's new update IOS 6.0.0(1). I know iPhones need a data plan for pictures, my iPhone and my sisters iPhone don't have data they're prepaid. She could still send messages but that's because she hasn't updated and her phone is a iPhone 3gs. Does anybody know a fix? Is it the DNS? If you know please help.

"Contact your network and ask them. Some settings made need to be re adjusted. "

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