a question for AT&T sim card swappers [USA]

[samster]

those of you on AT&T with swirly working successfully: at some point you had to swap sim cards. my question is after you swap the card to the old phone, add the regular messaging plan, and get MMS working on an old phone, and you then swap the card back into the iphone are you paying for medianet or do you still have your iphone data plan?

when i got my CSR to add the regular messaging plan, she took off the iphone data plan and put me on medianet pay-as-you-go. then swirly was working but i was worried about voicemail and data charges so called again and had them put back the iphone data plan but i was able to keep the regular messaging plan. and then swirly stopped working (recieves MMS but wont fetch) i would like to keep my unlimited data plan and visual voicemail. how did you guys have both regular messaging and iphone data but still manage to get MMS's?