Weird issue. I just got off the line with TMo support, and they gave me the following Access Point settings:
fast.tmobile.com in APN, MMS APN, connection name
http://mms.msg.eng.t-mobile.com/mms/wapenc in MMSC address.
I deleted the manual settings in Cellular.
I'm able to receive group texts, and I can send pictures as replies. But I can't send text. It is sent as a blank message to recipients. Had this working like a champ with Straight Talk. Don't know what I can do to fix it. Group text in settings/messaging is enabled as well.
Any ideas?