It works like this Bob, on the $15 base plan you can only call if you add a booster with a certain amount of minutes. Without it, the $15 base plan only gives you unlimited international messaging, nothing else.
Suppose you add the $25 booster, that gives you 500 minutes, both local and LD. That means any Canada call only costs you $0.05 per minute! The beauty is that as long as you keep your base plan active, those minutes never expire!
As both Ray and Dennis explained, on Koodo prepaid you never get a bill afterwards. It's like magic really :D