Happy New Year juanxi, and apologies for the delay in response here. I think you make an interesting point - you're correct that we should have IMEI use records and be able to match the line that the IMEI was used on to a specific account. However, legally we're not able to divulge that information to anyone but the primary account holder. So we can not offer unlock services to anyone but the original device owner, since we can't open up and review the account with anyone but them in order to check their eligibility, not to mention confirm or deny that eligibility.
As for how third party unlock companies complete the unlock process, I've honestly wondered that a few times myself, especially with iPhones as they're all unlocked remotely through Apple - we simply send the request and Apple completes the process. I can't speak to or advise you to follow that type of process, although I know we have some users who've recommended it as a solution when the phone isn't eligible to be unlocked by T-Mobile. I'm sorry to hear that you're unable to contact the original owner, as that's the only person we'd be able to approve an unlock for.