Still billed after temporally cancelling AT&T

I'm in international student and I temporally cancelled AT&T so I would not be billed by the months I was not going to be in the US; yesterday, I called AT&T to have my service reestablished and had US300+ in not paid bill, even though before I left to Brazil and I made sure to temporally cancel it.

What should I do now? 

Thanks,

- Caio