Account to collection months after properly closed

Hi, 

I have closed an account last year with Telus and paid off online my remaining balance.  We originally were with Koodo and switched over to small business account to save money, when we did they continued to charge us at Koodo while now charging us at Telus without our knowledge.

When we found out we tried to rectify the issue and it was too much to deal with so we switched. We still ended up with a large bill which I paid off online but now that I am checking up on my credit score I notice there was a charge after the fact that now has gone to collections and is ruining my score.  Does anyone have any experience with this?  This company has been extremely frustrating to deal with.