It looks like the answer is "NO".

Well, to elaborate a little bit, they will roam onto whatever cell phone network is around, but they won't let you make a cheap call over Wifi.

There is absolutely no technical reason for this to not be possible, just that it seems like they want to make sure that you pay their roaming fees, rather than saving yourself some money by going onto wifi and making cheap "international" calls over the internet.

Now, how do they know this? Well, they can tell by your IP address that you're in another country. They can't tell exactly where you are, but generally, IP addresses are given out on a country by country basis, so they can tell. This is how those websites can have ads that say "Find a car deal in Toronto", because they guessed based on your IP that you'd be in Toronto, but they can't pin you down to a particular address.

Apparently you can do this with T-Mobile's Hotspot or whatever they call it service that works the exact same way.

Now, there is a workaround for this sort of thing. You could have a router set up a VPN connection to a router/computer in Canada, and then you'd actually have a Canadian IP and they think the call would be coming from Canada, but that's pretty technically difficult (but by no means impossible). If you, say, live and work in Canada and some other country, something like this may not be a bad idea to set up.