I called to cancel my phone number and the billing is still coming. I already left the U.S. What can I do?

If you were using prepaid, you would still get a reminder to refill your account. That is not a bill. Prepaid is not required to refill there is no penalty for not refilling other than lack of service.

If this was prepaid service you're supposed to manage it yourself by logging into your prepaid account and turning the autopay switch to the off position.  Calling AT&T is not the correct method of canceling prepaid service.  Prepaid service stops when you stop paying for service.  No penalty.

If you were only in the USA temporarily you should never have been put on postpaid which would require a phone call to 611 to cancel.  

Hello @SiraprapaD, thank you for reaching out to AT&T Community Forums. We're here to address your concern.

We know that this is something important to you. We'll help to get this sorted.

As mentioned by ACE @formerlyknownas, please try to reach to out customer support team at 800-331-0500 or contact 611 to get your postpaid service cancelled.

Let us know if you have any other questions.

Thank you,

Mike, AT&T Community Specialist

Well since 611 and 800 don't work outside the USA perhaps the long distance number might be more helpful in the event they are actually on a post paid plan.

+1.314.925.6925