I cancelled my service with ATT due to several problems with a store clerk and a service overcharge

I had an overcharge applied to my April bill which was due to data use while on a ship.  I never used any data while on the ship and disputed a bill over over $1100.  After several weeks I finally called and found out that that day they applied that paid $1100 to my bill rather then refund the money.  This was the third straw that broke the camel's back.

I have gone to another carrier and have attempted to call and go online to find out when ATT will acknowledge that I no longer have their service and when will I get my $1100 back. The person on the other side said that they would mail me a check.  He did not know when and if I will get an acknowledgement of cancellation and credit owed.

Any ideas???