[smtddr]

If you're not using the AT&T sim, then you can never incur overage. I don't have the data to back it up, but I'm pretty sure overages are a significant part of their profits. If I'm wrong about that then my argument falls apart and I accept defeat.

[DanBC]

That just sounds like they are miss-selling their product and they should face some kind of regulation to prevent it happening.

Caveat Emptor and everything, but routinely exploiting ignorant customers should not be a business plan.

[kalleboo]

If you're not using the AT&T SIM, you're also not putting any load on their network, and they can oversell it more without spending any money on network upgrades.