Statue of Limitation on promissory notes

I recently received a letter from a collection agency telling me that I was a co-signer on a account from November 2001. This has never been on my credit report and this is the first I am hearing about this in 7 years? I called to see what it was. Now they are telling me a friend I co-signed for 7 years ago is refusing to pay them because she feels that she has paid the loan off and I am liable for the debt. They wanted me to pay the $3000.00 by the end of the month and they will not come after me for the full balance of $4790.47. I have checked my credit report and they have inquired on my report? Are they able to put this on my credit report? In addition, when I called them I called from my work number and they had caller ID therefore, about a week later they called me and asked if I was paying the $3000.00 by the end of the month. I told them never to call my work number again. Then about three weeks later they called my work number again and asked again if I was paying. I told them that this was not my debt and I was not paying this and to not call my work again. Is there anything I can do to prevent this from going on my credit report and to make them stop calling my work