The below article at Forbes asserts that WiFi calling will use cellphone plan minutes but not cellular data. This type of account detail is best understood by reviewing your plan, perhaps via support from the carrier. "Seamless" (as opposed to seamy?) WiFi Calling must be supported by your carrier, plan and phone. On my TMobile Android it's available as a preference and the default dialer app can use it when configured and suitably connected via WiFi. Apps like WhatsApp can make voice calls over WiFi as well.  In my experience the phone will not automatically choose WiFi calling unless configured to do so. 

https://lifehacker.com/tech/what-is-wifi-calling-and-how-to-use
https://www.forbes.com/advisor/business/software/what-is-wi-fi-calling/