SOL in SC - 6 year old charge off

I made arrangements on a revolving account in Dec 04 that was confirmed in writing by the creditor. Before I could make the second payment their system charged off the account in Feb 05. I contacted the creditor for months trying to make arrangements to settle the debt and was told there was nothing to do because the account was charged off. They have never called me to settle the account in 6 years. During the process of applying for a new mortgage this year, the application was put on hold because the creditor shows the old account was involved in bankruptcy proceedings in 2005. That was an error on their part. I had to contact them to correct my credit report. It was changed from bankruptcy to charged off with an outstanding balance then a month later disappeared from my credit report.

This is the only debt that I have never paid in full and I tried for a very long time to settle it but was always told by the original creditor that there was nothing to do so I left it alone. Yesterday I started receiving calls from a third-party collector about the unpaid debt. I did not acknowledge the debt . . I explained that I was at work and would have to call them back.

How should I handle calls from the third-party collector? I would feel differently if this were the original creditor contacting me, but a third party calling on an account that was charged off 6 years ago and no longer shows on my credit report concerns me.