I got this phone from ebay and it says it is on T.mobile network, but then when i tried three all T.mobile simcards on it, none of them works. it says "Sim lock".

The person to whom i bout it, said that it works on him before sending, and t.mobile said as long as the phone is at their networks, they dont need a code ot it to work for. I do not know what to do??? can anyone help me to unlock it?

The details:
Samsung SGH-E900
IMEI:354711017470109

many thanks!