AT&T does cover Canada (I asked at the AT&T store.
When I first move d back to WNY after y stroke, I got a notice on my phone that I reached my minutes limit even though I ha unlimited text and data. Called them and they told me it was "ghost roaming". I live in Cheektowaga and can see the M&T tower downtown. Was never less that 5 miles from Canada, but my phone was pinging the towers in Ft. Erie. They said it happens near Mexico too. After that, no problems.