Samsung blackjack mms/aim help. plz help!?

so i have a samsung blackjack and each time i recieve a mms or try 2 send one it says unable to connect. verify your profile or try again. Yes i did go to the at&t website and the support section i went to the device help and under it the mms and internet connection i noticed that it said the access point and everything was att but on the website it said cingular. ex. wap.att; on the website wap.cingular. and one more question does it matter if u change your phone without telling cingular?? like i had a flip fone bfor and i switched to my bros samsung blackjack. thanks