I think you should give a discount to the main account holder for every phone added to the account. Families with children now have multiple phones in the home and offering a discount to the account holder(which is usually the parent/ bill payer) for each new signup in his/her household will encourage the parents to go to Koodo for each child when they are ready for a phone. If you offered a $5 discount perhaps for each addition phone on the same account a parent with three children would receive a $15 discount off his/her bill but you would have three new phones on Koodo service! A win-win situation for everyone.
[b]I do believe this is a prize winning idea for your contest :)