WiFi calling iPhone

[JMI]

Please can you help? I've searched the boards but can't find the answer to this question.

I need to call the USA to a company and I know I can do it free via WiFi calling. I have changed the settings on my iPhone to enable WiFi calling, but how do I know it will use WiFi free rather king up a massive bill for calling the States?

How does the phone know to make the call using WiFi, or will it do it automatically when I am using WiFi, so example at home?

Im getting myself very confused and really would appreciate some help.  If you are kind enough to explain, please assume I know nothing and walk me through it. 

I can use FaceTime and WhatsApp calling free, but the number I want to call is a business number so I can't use either of these facilities to make the call.

thank you so much

Jean 

[Cleoriff]

You are lucky @JMI . The nearest I've come to seeing them is at Longleat. I have a house with loads of framed pictures of Big Cats, Snow leopards, Siberian Lynx, Cheetahs, Leopards etc, etc, etc. Also watch pretty much every programme on big cats. Love them xx

[Bambino]

@JMI If you download Skype and buy some credit it will be cheaper to call the US that way, rather than using your mobile.

[JMI]

thank you for the advice, I will look in to that 🙂