AFAIK, the only place that can unlock a T-Mobile iPhone is T-Mobile unless there are people who can access to their systems and do it. I think there were some unlocking services for Sprint on Ebay but now I don’t know.
The way it works with the iPhone is that the carrier submits the unlock request to Apple. Only Apple can unlock the phone, but they cannot do that without a request from a carrier. So, Apple inserts the IMEI into the unlock database they maintain, and an OTA profile update is pushed out from the iTunes servers. The next time your iPhone phones 'home' the update applies the profile and your device is unlocked.
My point in mentioning all that is that even if someone COULD access T-Mobile's systems, they'd need to know how to submit the request. Because the carrier cannot unlock the iPhone, they can only request that it be unlocked.
PS…regarding Sprint. I was a Sprint customer for 16 years (1999-2015) and I read up a lot about unlocking from 2012 to 2015 when I left. That's because I had such horrible service during Sprint's LTE rollout I wanted to see if I could get my phone unlocked.
But Sprint didn't start allowing unlocks until February 2015. And even then, they had an internal team that was EXTREMELY good at finding employees who submitted unauthorized unlocks and firing them. The price of a Sprint unlock from a third party service then was very high because the risk of losing an inside contact for unauthorized unlocks was also high. Or, it was simply a scam because no one could get Sprint phones unlocked. Most of the time it was the latter.
PPS. There was a short time period where Sprint discovered that international unlocks for the iPhone were defaulting to full unlocks. Sprint systematically found those people who'd gotten full unlocks through this method and had Apple remove them from the unlock database. Basically, they rescinded the unlocks. Sprint was extremely draconian about unlocking.
