You can get free call back from anyone you want simply by sending a link! The receiver has only to click on the link and the call will start automatically.
On your iPhone: You can send a free call back link simply tapping on a contact (who must have an e-mail address associated) and then tapping on “Free call back”. Your contact will receive an e-mail with a link inside: clicking on that link he will be able to call you for free.
You can also send a free call back link to an e-mail address not in your contact list tapping on “More” and then on “Free Call Back”.
On your PC or Mac: On your contact list, simply click on "Free call back" button.