Skype needs Wi-Fi or 3G Internet access. If you have it, you can make calls. You don't need a calling plan to use Skype, you can use a data plan for it.

So you can replace your calling plan with Skype. But make sure you know the rates for calling the U.S. and Canada.

Also I recommend to take a look at SIP providers. For example, search for betamax and compare their prices. Usually they are much better than Skype rates, and some countries can even be free.