I have to commment on this, wifi calling actually does use your minutes, It gives you a message everytime you open up the wifi calling and informs you that wifi calling uses plan minutes. Of course Tmo to Tmo there is no charge or nights and weekends but your daytime minutes are still used while using wifi calling.

You also forgot Tmo Data Pricing in that, Tmo offers 2gb data for $20.00 while AT&T offers 3GB for $30.00. however with at&t if you go over the data it is $10.00/GB overage and Tmo just slows your speeds.