Academy Collection Services

About three weeks ago, I began to get calls from multiple phone numbers on my cell phone. I answered the first one, and it was an automated voice asking me to "please hold for the next available representative". Therefore, I hung up. I Googled the number, and it came up as Academy Collection Services. Since the initial calls, I have received multiple calls from them, each from a different number.

After the first call, I ran my credit report from 2 of the credit agencies. Both confirmed what I already knew - that I have NO accounts past due. My guess is that this agency is trying to collect on a debt that was incurred in February 2000. This debt was sent to collections as I disputed it, and I finally settled in August 2006 (yes, I know it was beyond the statuatory time!). As the debt was settled for less than the full amount, I am thinking this agency, Academy, bought the remainder and is now trying to "recollect".

My question is twofold:
1. My understanding is that since this debt was paid/settled, the remaining amount CAN NOT be sent to yet another agency. Is that correct?
2. I live in Colorado. I have found multiple statutes that state either a three or six year limit. This was from a medical bill with multiple errors (hence my refusal to pay). IF they can collect on the remaining amount, what is the statute for that time?

I have tried to contact them back, and they have yet to return the call. I have no account number to reference, nor have I any idea what this could be about other than the scenario noted above.

Any help is appreciated.