My most recent unlock request from last month was granted despite the device has never been turned on with my prepaid SIM inside. I ordered an used smartphone from a third party, had it brought to me abroad, and then I requested the unlock code (over skype). The code was delivered to me by e-mail in 3 days.

I think the difference was that my device was used, having been used by another customer on T-mobile's US network. I imagine T-mobile does not want people buy new phones, unlock them, and then resell them at places like Sin-tat. Even at unsubsidized prices, most carrier-branded phones are at a discount compared to the international versions.