Your lucky only your calls drop, I think after I turned on Wifi-Calling in my (stock android 6.0.1) Verizon Note 5 it crippled the whole use of wifi! Noting Internet related works in my phone with wifi on now, but everything works fine via Verizon lte. I have eliminated all other causes of this problem (checked home internet connection, checked to see if it was my home modem/router problem, checked if my phone's ip was being blocked, "forgot" the wifi networks and reconnected again). I even Backup & Reset my phone. Everything worked fine for a day, but as soon as I turned wifi-caling on, same thing happened!