Law office letter says i owe money but it is not my acount.

Today, I received a letter from a law office telling me they have taken over a debt collection of almost $4,000.00 and lucky me, I can settle for $1,500.00. The trouble is, I do not have this account, I do not know what they are referring to. none of my personal information is on the letter, such as employment, social security number. Just a strange letter asking for money. The office does exist, I called and hung up. I wrote aletter telling this law office that this information is incorrect, or fraudulent. Any other things I can do?
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http://www.oskie.com/free-credit-help-sample-form-letter-letters.htm
send them the debt validation letter
then the follow up