You can find the wifi calling app on Tmo TW based ROMs. The is no separate app. It's baked in.
Hastily spouted for your befuddlement
I remember there being some WIP of a Wifi Calling framework for cm9, a long long time ago, for the mytouch4g (or maybe the galaxy s2, I don't really remember which), I wonder how far that got :/
It's certainly possible, it's just that the dependencies are such a pain to search through. And it's not like using wifi calling will not use your plan minutes (it used to, like 4 years ago, maybe more).
But yes, I still suggest TW for underdevelopped phones. Omni has no exFat support and falls short of CM, and both have terrible battery life. The last one I was on was actually Conch Republic 3D (F2FS), and it's INSANE with battery life (slightly better than my brand new debloated s5). The time it takes Conch to get from 100 -> 88 is the same amount of time it takes CM to go from 100->64, and honestly, batter life is something I can't compromise on.