I'm the OP. I heard similar stories - a friend got an AT&T bill for $1200 after getting off a plane in Europe (from US) and turning it on for 1 minute.
I'd bet my reputation that AT&T is bundling data packets across the border - and that this could be the basis of a big class action suit.
In my case, I was driving northward from Oak Harbor to Vancouver. Roaming was off on my AT&T phone - so there should have been no intl data billed until I turned it on for that one minute.
A roaming clearing house is an organisation that manages the billing between carriers in roaming agreements. These clearing houses exchange data in a common format (bytes per session) between telcos. It's almost certain that AT&T use one for their roaming agreements.
If pressed, AT&T would need to correlate three pieces of information - the clearing house data, their billing, and the canadian network provider's logs. Each entity is legally separate here - if all three logs line up, it's going to be hugely compelling evidence.
It's very unlikely that AT&T is gathering up your data charges and billing you as you go over the border - it would be supreme incompetence at a carrier of this scale. More likely, something's gone wrong at the clearing house, and you should press them for aligned, detailed records. (I audited telco billing systems for a few years.)
I think what he's suggesting isn't that the logs don't line up, but that AT&T may be purposely switching the phone to use the data roaming opportunity. Even though carriers gouge each other horribly on roaming, the marginal profit may be even more to the consumer. If this is true, it would be a scandal.
I'd bet my reputation that AT&T is bundling data packets across the border - and that this could be the basis of a big class action suit.
In my case, I was driving northward from Oak Harbor to Vancouver. Roaming was off on my AT&T phone - so there should have been no intl data billed until I turned it on for that one minute.