I think that most (smart) phones do not know how to create a seperate ip socket to send/receive mms over wifi. So they revert by default to the data socket provided by the carrier. Or they may not have the ability to switch between two different sockets automatically. If you do root your phone and change the default mms ip socket, you may lose all mms connectivity depending on the phone.

Hope this helps, Peace, Yo from: JJdaCool

I think that most (smart) phones do not know how to create a separate IP socket to send/receive MMS over wifi. So they revert by default to the data socket provided by the carrier. Or they may not have the ability to switch between two different sockets automatically. If you do root your phone and change the default MMS IP socket, you may lose all MMS connectivity depending on the phone.