An advertiser pays to show a video of their business or product. The money they paid for that Advertisement is what pays for your call.
You want to make a call and decide that you don't want to pay for this call. You dial the number you are calling and then select the outgoing free-call function. A video ad will play. Once the video is complete you will be connected for your FREE phone call. (Of course a user can override the free-call option and the call will be connected immediately.)
Your phone receives a signal that a call is coming in and displays the name &/or number of the person calling. At that instant a video ad begins to play. If the user wants 100% of the call to be paid for, they must watch the video before answering the call. (Of course a user can override the free-call option and answer the phone immediately.)