So let me get this right. I can go get my teenager's iPhone and make a call with my iPad right now? Mike G said it best earlier: you will need an iPhone and an iPad to make a call on the iPad.
if both devices are running iOS 8 right now (and are on the same account) and you go and get your teenagers iPhone then yes you will be able to make calls with your iPad right now.
https://www.apple.com/ios/ios8/continuity/