WiFi calling not able to see in iPhone 6

Wi-fi calling is supported by your carrier. Check with your cellular carrier about implementing wi-fi calling. See this, Make a call with Wi-Fi Calling - Apple Support

That should be found under Settings>Cellular, however I don't have an iPhone with iOS 12 on it to check. Again, you need to ensure that your carrier supports wi-fi calling and they should be able to help.