Sent my final bill to collection agency without co...

Sent my final bill to collection agency without contacting me at any point. I had thought I paid it at time of cancellation, but apparently not and I take responsibility for that. They had my phone number and a forwarding address. I actually opened up a new account 5 months later and paid 6 months of bills on time for that account before I received a notice from a collection agency a year and 3 months after the first account was closed about the previous bill. I was never late once in two years paying their bill. Hard to pay a bill you never receive or know existed.
Customer service did nothing but defer and make excuses and say that it was not their problem or issue. I admitted I owed the money, paid it...thru the collection agency, but wanted some answers about how it got to that point and received nothing but I m sorry sir but we can t guarantee that we know you owe money.
Sounds like if you never want to pay just keep opening new accounts!!