Money I shouldn't owe

I canceled my at&t account back in February of this year while I lived in oklahoma, when I tried to return the gateway the ups office said they wouldn't take it. I tried everyone in the town I lived in but nobody would take it and deliver the gateway like at&t said they would. Soon after I moved to California and kept the gateway in a box until just today when I found a ups store that would deliver it. I called at&t to talk about my bill and to set up a new account ,since I have used them for well over 4 or 5 years, they told me it would cost 163 dollars,so I told them I just returned the gateway and was wondering if they could come down even a little bit what they told me was that I had been using the gateway up until early may and that I returned it near the end of may. I explained to the lady that it couldn't be possible since the only internet gateway I ever had with at&t was just shipped and that I had paper work to prove it, she proceeded to tell me that I already returned the equipment and that all the charges on the account where for usage. Can someone please help me figure out how I can usage up until may in Oklahoma when I've been living in California since late February and cancelled the service then!?