How do I get Wifi calling with an unlocked phone, I use AT&T?

Short answer? You can't. :(

Long/complicated answer: With the right hackery and using an ATT ROM (which might potentially brick your phone because things aren't obviously the same internally), it might trick the carrier into thinking it's a carrier-unit and allow WiFi calling.
 
On T-Mobile, all features (including WiFi calling) work on the unlocked model. But other carriers are different.