We used to have At&t. The latest plans should already include data, calls and text in
Canada. Our hot spot device (mifi thing) is now on another
service (Net Buddy) and isn't supported in
Canada.
We switched our phones to Cricket recently (which uses At&T towers). It has unlimited data, calls and text in Canada when we're there. The only catch is that they don't let you hot spot phones unless you pay more. Even then, we found our phones weren't supported for hotspotting under Cricket. The
work around is that we installed PDA Net on our phones and can hot spot them when in Canada, and Cricket seems to be none the wiser.