Unfortunately this is one of the features that is carrier-dependent and it's up to the carrier you're using to make it available to unlocked devices or not. It's clearly stated in the phone specs and description when you buy, and fairly well documented on reviews and forum posts.
Like you mention, flashing a software ROM from a carrier could help, but it's not without its risks, obviously. IF you have no idea what these rooting tools are or don't feel comfortable following the guides at sites that specialize in rooting/modding, then I would advise you NOT to dabble with that. You might render you device unusable.
You can call Verizon and verify if they support WiFi calling for an unlocked S8 (might need to send a carrier signal/message or config file) or not. If they don't, though, that's about as much help as they'll be. They will not flash your phone or load new software.