I am in a similar boat - I bought the phone from Ebay and it was T Mobile (brand new sealed). No service was able to unlock it for me so I called T Mobile and opened an Pre-Paid account. I put $10 on and used this while in the States to show it registered on T Mobile system.
I called to ask if it was registered on their network and they said yes. When I contacted T Mobile today, they said they can unlock it today if I put alteast $100 in refills. So once I did that, and called back - they then told me they still could not unlock it as my IMEI number was not registered in their system and my phone was not showing as a T Mobile phone!
I escalated the issue to the Support team and hopefully hear a response in the next two days. I told them that I did all they asked me to do including calling several times to verify the phone was with T Mobile so they either unlock my phone or refund me the $100 I paid.
Hey man, I am doing exactly this, I asked a friend to get the sim card and bring it, but ETA is next monday (Jan 11th), so I called the t-mobile to make sure it would work. So they said, activate the sim card on the prepaid plan, leave it in the phone for at least 72 hrs and then pay the $100 to the plan, after the 72 hrs call them back and ask for the permanent unlock, they confirmed that the unlock would be right away. So now that you are saying this I reaally want to keep in touch with you, so I can understand how it went. I have until Jan 14th to return the phone to eBay. So if it does not work for you, so, it probably will not work for me. Hope you will get answer soon m8.