How to get AutoPay payments to stop on cancelled account?

I am living in New Zealand. In August when my family was in Mississippi, we took out a plan for my son for a mobile phone which was $60 per month while he was at university in the US.  He only stayed a semester and came home at Christmas.  After he decided not to go back to MS, I contacted AT&T to cancel his account. I did this via live chat but there was no login required.  I was surprised at how easy the process was and asked the customer service lady whether there was anything further I needed to do, but she said it was all done for me.  Being live chat, I unfortunately have no evidence of this as I did not think to screen shot the chat window at the time.  I looked at my bank statement today and was shocked to find out I did not notice the monthly charges are still being applied to my credit card. I have tried to get in touch with A&T but am in a loop going nowhere:

• Tried calling the Vicksburg MS store - got an automated service that eventually warned me that due to call volumes I would have to wait a long time to speak to a human (I am calling internationally, so no good)
• Tried calling the international support number - got an automated greeting asking me to enter the PIN or last digits of social security number (have neither of those)
• Tried live chat again - now I seem to need to have an account to login before chatting with anyone - tried to create an account but was asked for local billing ZIP code - I am in NZ and our ZIP codes are not the required format so not valid... cannot create an account and cannot chat (in any case, technically I am no longer a customer since the account should be cancelled anyway)
• No email contacts listed on the AT&T website

How can I possibly investigate why the account I cancelled is still continuing to bill me and put a stop to it ASAP?  

All help most welcome!

Virginia