Send the debt validation letter because this is the first legal step by which you'll be sure that the CA is doing legitimate collections. Send the letter through certified mail return receipt requested. Once they have received the letter, they will be forced to give you the details. If it's a mistake, they will have to correct it.
In case, if they try to prove your avoidance in paying the debt and file a legal case against you, the debt validation letter must be your important defense. Send a dispute letter also mentioning that you are not aware of the account ever used by you. Show the dispute letter when the judge asks for it. You have to cover your grounds strongly while dealing with the debt collection companies.