Good news! 911 has always been handled by the local operator, so there is nothing that needs to be done on your end. 911 will be default always be picked up by the nearest emergency provider, no action needed on the user end.
This isn't actually something limited to T-Mobile, it's how all carriers handle 911, so you won't ever need to worry!
The only exception for this is WiFi calling, and in that case you just update the e911 address with the steps here Change your billing and e911 address This is only for calls made to 911 on WiFi, any call made on network is automatically routed as mentioned above!
If anyone lives in an area where there are separate little towns in close proximity to each other, it's a good idea to program the local PD/FD number into your phone. Not only do you not want to call 911 to report a stray dog, when you do need to get help, the 911 call is often handled by a tower outside your town, which is routed to another dispatcher, who then has to find out where you are and reroute the call to your town.