Past-due bill resolution involving IC System collection agency

I need to pay a bill on an old account that has been closed and sent to collections. The name of the collections service that I see in my credit report is "I C System".

I have read that there can be problems getting a complete resolution when IC System are involved, so I don't want to simply pay through the AT&T web site (the balance is still payable, apparently). I need help from a representative who can make sure that the open issue with IC System is closed and that the derogatory mark on my credit report will be removed.

Thanks in advance!