As per the laws in the fdcpa, the collection agency is required to send you the debt validation or verification details when requested by you.
Since you don't have the proper account information, you have to wait till the company sends you the correct details. The federal laws allow you to refuse payments till the debt is validated. Make sure that you send your letter through certified mail with return receipt requested. In case, if the matter reaches the court, you should be able to prove your defense by showing the certified receipt slip mentioning the date when the letter was sent.