Friend of mine has the Galaxy S5 on Verizon and was asking me about wi-fi calling. Did Verizon activate wi-fi calling on the S5, or is it one of those things that might be included in the Marshmallow update when they finally push it?
IIRC it would need a software update in order to actually put the software on the phone to work with WiFi calling. So I would say it would need the MM update.