Unlimited Voice and SMS to other carriers is expensive. Look at Twilio's pricing or Skype's pricing, or any other voip provider. Text and calling to your own network is essentially free, but off network gets expensive. Granted, I don't know how big of a cut these voip providers take, but I can't imagine it's that high.

Put it together with usage data, you quickly get over $19: http://blog.nielsen.com/nielsenwire/online_mobile/u-s-teen-m...

Edit: Just want to point out, I'm not saying this is a bad bet to make. They can attract a ton of consumers with a super low cost, and if those consumers do most of their communication in-network, eventually they'll be in the green.