Here is what you need to do: under the messaging tab open the message settings button. Multimedia Msg, down at the bottom is message center. There yoou will see the ATT MMS. Add another and name it T Mobile (I did). enter the MMSC URL- mms.msg.eng.t-mobile.com/mms/wapenc. ubnder internet profile enter tmobile
(settings are epc.tmobile.com) you will have to add this profile to the internet settings on hte phone as well.
Hope tis helps. I did it to my son's phone and it worked.
Here is the link i used...http://support.t-mobile.com/docs/DOC-2013