Comcast Poor customer service. Call from collection agency for Current bill. Nationwide

I recieved a call from collection agency today on the weekend early in the morning for a current bill of Comcast. I recently disconnected my home internet because I got a new connection from comast for my home business. I paid my all dues but was told that I will recieve a partial bill, which I recieved yesterday ( on 9/12/14 with the bill date 9/7/14). It shows current and nothing is past due. I always pay my bill online. I tried to pay my bill online but my acount was disabled.

I called comcast today and talk to Liz at billing and paid by myll $36.60 and asked her why comast sent my file to collection agency for a current bill and without sending me any letter or calling me on my phone. She said she dont know who send the file becuase there is Millions of people working in comcast. I asked her to transfer me to someone  or give me phone no of someone who can explain this issue to me but she said she have no idea and can not help me.

I dont understand how comast can send a fle to collection agnecy for a current bill $36.60 for the customer who was with them for last 6 year and is still have business account with them. I am very frustrated and want to disconnect my current business account but comcast has monoply in our area. I dont have any other option to go with some other company.