wifi calling means if you are away from a cell tower (no signal) but you are connected to wifi, then your phone will work just as if you were connected to tmobile cell tower (make and receive calls)
this feature is so great when out of the country. on a cruise in south america recently i was able to connect to wifi at a starbucks and make phone calls. internet is universal, so its just like you are still in the states making a phone call. no roaming or anything
by default the 640 is set with wifi calling on and set to prefer wifi.(this causes problems with some things) lots of people turn off the feature, personally i left it turned on, but changed it to prefer cellular. and that fixed everything.