I have had a similar problem when I first got my unlocked phone. I have T-mobile as well, and once they find out you have a phone device not purchased by them, they are not very helpful - they will give you their default mms settings(same ones listed on their site). I tried those settings and they did not work on the particular unlocked phone I had. I did some extensive web searching and came across a few settings that worked with my phone. I will list them here. Hope they work for you!
*MMS Settings: Home URL -
http://216.155.174.84/servlets/mms
*Protocol Type - WAP
*WAP IP Address - 216.155.165.50
*GPRS Access Point - wap.voicestream.com
*GPRS Username - Leave Blank
*GPRS Password - Leave Blank
*Authentication Type - normal
*T Zones -
http://wap.myvoicestream.com
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