It's probably a notice that a phone on your Tmobile account has been approved for an unlock code or is eligible to be unlocked.
Nothing about hackers that I've seen.
As mentioned, it is probably to have your device SIM unlocked so it can be used with another carriers SIM card. If your device is ever lost or stolen, report it as soon as possible to T-Mobile to have that device blacklisted, so it can not be used on any carrier within the US or Canada.
To add, it sounds like you are also asking about SIM swapping. Just do an internet search on that term--SIM Swapping--and read. One of the things you can do as far as your T-Mobile acount is set a PIN so that anyone calling would have to know that number to make any changes. You can set the PIN at T-Mobile, or if you call customer service I'm sure they would help you.