Now that I think about it, this could be considered a lottery if you're in the USA.
If there's any payment whatsoever, it's not a prize, it's a lottery. You're purchasing something in order to win. In this case, phone calls. Cost can mean anything, though, including an unreasonable amount of time (say, review this thing and win... but it takes an hour to review it. That's a cost)
Prizes need to be:
A.) A lottery, in which there is a payment, anywhere in the process between entering and winning. Any payment, paying affiliates, paying a nickel, paying anything. If you lose a penny as a result of entering, that's a payment.
B.) Games of skill.. The best _____ wins. The person with the most _____ wins. A certain skill is utilized to win the contest.
C.) Completely random drawings.
B & C both must be free to enter. Creating a lottery is illegal in something like 48 states, as they already have their own lottery and the proceeds likely aren't going to charity.