During conference calls, you will be charged only for the participants whom you add to the call. So in a particular con-call if you add 2 people and one of your other friend adds someone else, you will only be charged for 2 people, and not the third friend (whom you didn’t add).
So, if you are a smooth talker, you can get all your friends to add participants and that way you will only be charged for one call; neat, right?