Re-collection of settle debt and SOL Medical debt

I settled a medical debt for 50% of amount owed in 2007. The debt was initially incurred in 2000, and contained a number of errors. I wanted to do what was right, so I paid the undisputed portion when I had the money. I received a fax from the original collection agency confirming the debt was settled in full, and also have a copy of my Visa bill.

Fast forward to 2010. This matter which I have long since forgotten about has come up. I am getting calls from multiple collection agencies trying to "re-collect" on the amount that was written off.

Since the last activity was in 2007, the statute "reset" to another 6 years - although not sure if medical fall into the 6 year or 3 year category? Could someone please confirm which statute applies to medical bills?

Second - is it legal for another collection agency to try to re-collect on a settled debt for the remaining amount of the original debt? If so, what are the statutes to support this as I can not find them? Or, do I just send them a letter stating I will not pay a settled debt. I don't think they can re-post this on my credit record since the original debt is 10 years old. However, my score is in the 800's and I really don't want to jeopardize that.

None of the new agencies (3 so far) have yet to send me any documentation. They do not have my name right when they call, and it is a work cell that I do not take personal calls on. So far, only one VM has been left for me. I live in CO.

Thanks.