In some situations, especially with prepaid phones or some smaller carriers, WiFi calling isn't allowed. When WiFi calling first came out, some carriers charged a monthly charge for it to be enabled, but since it's become a more standard feature that shouldn't be a problem. If in doubt, check with your cell carrier. 

However, if it turns out you can't use WiFi calling for some reason, the best work-around is to use an app that allows making phone calls. My personal favorite is Google Voice (free), but Facebook Messenger also allows calling phone numbers through the app. 

For instructions to set up Google Voice, follow the steps found here