Its not that its in an iphone.. its that its on an "iphone plan" which they see whether or not there is a sim in that phone or another.. What you would end up doing is, when you call and tell them, "Its not in an iphone" they will say.. "ok, let me get you out of this iphone plan" and they will do that. .then your sim will not have the iphone data which makes your phone close to useless..
Then when you call them to reinstate the iphone status on the account, the MMS block will go right back
Then when you call them to reinstate the iphone status on the account, the MMS block will go right back