according to CNET article:
"For domestic calls, it doesn't cost any extra as such. But making calls over Wi-Fi can come out of your regular minutes allowance depending on your carrier and your phone plan. Be sure to read the policies of your carrier to see if any of their potential restrictions and charges relate to your situation. Click the following to read the restrictions for T-Mobile, Sprint, AT&T and Verizon."
https://www.cnet.com/news/what-you-need-to-know-about-wifi-calling/
I checked Verizon's policy on Wi-Fi Calling, and it states:
The Wi-Fi Calling feature is available at no additional charge, and is included in your existing monthly voice plan. Your Wi-Fi calls to US numbers, regardless of where you are located, are also free of charge (except for 411 and other premium calls). Wi-Fi calls to international numbers are billed as "International Calling While in the US" (also known as International Long Distance). If an international rate plan exists on your account, international calls are billed at those rates. A Wi-Fi Calling icon will appear on your screen during the call to confirm that it is using Wi-Fi to connect it. If the call is using the Verizon or a roaming network, standard voice charges apply.
https://www.verizonwireless.com/support/wifi-calling-faqs/#cost