ideal wifi calling like tmoile

first off i would like to say im by no means the most tech person from it. but i have been know to to mod my devices most times cuz im bored and have nothing batter to do. but enough about me and lets talk about my ideal. i have the nexus 5 so i been looking in to how to have wifi calling. two things i notced was a post back when tmobile dodnt have unlited calling and text that wifi calling was taking away there mins and and other was someone tryied usibg his wifi calling and to call some one and was forward to tmobile.
so when someone is using wifi call it most connet to tmobile sevres vai the internet from my limited knowegle it make act some what like a vpn.

i was wondering what about unlock phones from tmobile or the ones that uses it servse does the wifi calling still works or does it call tmobile or just not work.

so what i think it does connect through to the comp throw the internet . it may work just like the free calling apps but insted of the app its just built in to the rom and connects to tmobiles sevres. we are talking software. some one some sould look at the codes of the apps and tmobiles roms. oh i was talking to tmobile and tell said they had to change the code on differnt froms but then again the people from tech supprot really know know what there talking about the just read of the screen and tells them what to tell u.

some one i know who has metropcs and there and can now do wifi calling so you sould be able to compare the roms and see what makes it. of and it most likly uses the sims card to help pull up the account. so i was thinking the costome rom could use the apn and the imei or sims card number to connect to the comply you are using at the time.