I've got an unlimited SMS/MMS plan from AT&T. SMS works just fine, but I can seem to send or receive multimedia messages. When I try to receive one, the message says "The message from (insert sender here) has to be downloaded." and "To download the message press 'Download'". When I press download it says "Alert. Could not send and receive all the messages." When I try to send one, it says "Alert. Could not send all the messages." Do I have to have a data plan to actually be able to send/receive MMS?