Part of Wi-Fi calling is text messages also, you are on the T-Mobile network the only difference is that rather than a cell tower to a dedicated data line (backhaul) you are using an internet connection to connect to the T-Mobile network so essentially sees it as any other cell call for the most part anyway.
As for your concern over phone companies "tacking" on other charges you apparently haven't seen the rant that John Legere CEO of T-Mobile has been on for the last six months about abolishing overages, he even hired an airplane to sky write over Verizon and AT&T's headquarters saying they should abolish overages. The only charges besides, line, extra data and taxes +EIP (Phone payment if you finance one) would be for any premium service you sign up for or app you authorize to charge your account. The only other thing would be international calls if you made one so you would know you are incurring the charge but unlike AT&T or Verizon where if you are near Canada or Mexico and accidently get connected to an international cell or call a Canadian number not realizing it is Canada on T-Mobile that if free on Post paid accounts.
As stevetjr explained, WiFi calls still go through the PSTN. The only difference is that you are accessing the T-Mobile network via your wifi rather than a cell site.
If you want free calls (i.e., not counted as minutes), use a VOIP service like Skype or Hangouts.