As long as you are using VoIP (Voice-over-Internet Protocol) call applications such as Whatsapp, Viber, Messanger, Hangouts, Skype and more, you can use the Travelers Wifi network with no additional roaming fees. You can also check your home mobile operator, if Wifi-Calling is allowed. Learn more about Wifi-Calling here:
https://www.travelerswifi.com/blog/all-you-need-to-know-about-wifi-calling/