I talked with Sprint about international calls before signing up on this one - you can not get any plan for discounted intl calls with this promotion - their call charges start at $1.28 per minute to Canada! (as another poster noted)
So, if you want the 40,000 you need to have 18 months of calling card calls for intl - I guess that is how they make their money.
Guess it is too late for the NW 40,000 miles deal?
What I do for international calls is use a dial around service. I like this as it has no monthly mins. and reasonable rates for most places. You are billed , so you do not have to worry about recharging and running out in the middle of a call and such like with a calling card. For Canada it is 6.9c if I remember correctly.