Interesting.
Did a bit of searching around. Apparently the Android version doesn’t matter so much from a certain level on, I found references to it talking about Android 5.1.1 and Android 6.0.1.
The big players in Germany (Telekom, Vodafone, O2) restrict this feature to certain mobile plans, Telekom additionally demands “Telekom Firmware” (so perhaps only branded phones, I don’t know). I don’t know how it is at your place.
If your mobile plan supports this, you could start the Dialer, dial *#*#4636#*#* … this starts a “Test” menu. From there on go to Phone information, there you’ll find a switch for WiFi calling.
If it doesn’t work, tap the 3 dots (right at the top) - IMS service status (or something like this, German locale here) … This should tell you, whether this feature is available or not.
I guess you’ll have to work with your provider a bit on this.