I get 55Mbs on ATT in my house in Fort Worth, TX. I get 100Mbs outside by my house. Inside work 2-8Mbs depending on where or if I force my Galaxy S6 from LTE to 4G. 4G pentrates better. Outside at work I get around 25Mbs. At work every carrier has speed/signal issues in the middle of the store. I think all of carriers are limited in the power output since there is a militry base across the road.
Att is adding wifi calling to more Android devices. The LG G4, LG K10, Samsung Galaxy S6 Active, S7, S7 Edge, and upcoming Note 7 support wifi calling. iPhones 6 and newer support it also. When using wifi calling it doesn't use minutes if you are on an old minute plan.