That's why I don't use any custom ROM with my T-Mo phones and always buy T-Mo branded phones - I live in a comeplete deadzone - and WiFi calling is usually one f the first things that go in custom ROMs. I used to wonder why other carriers didn't add theur own versions of WiFi calling since it still counts against the plan's minutes.
Thank you guys. Actually my phone is the Developer's Edition. I'm still in search of a ROM that is stripped of T-Mobile bloatware and running as stock as possible with the Wifi Calling feature. It seems like I am unable to download/flash the firmware upgrade as my Model ID isn't matching either for 5.14.531.11 firmware. 5.11.401.10 seems to work fine with this ROM though.