If you were to call Cingular support and tell them that you will need to recieve MMS's from multiple Networks, they might confirm that it is not possible. Or else they may say that you could but in reality it is not so. Secondly due to the way an MMS gets transmitted, and if you are in transit, the message recieving will fail. There's a lot more to it.
What are the issues about MMS delivery? cont'd