No responses to PFD letters. Now what??

I sent out PFD letters to four different CA's in February. I still have not gotten a response from any of them regarding the letters. I'm still receiving the same generic letters from the CA's that I was receiving before, but nothing in response to my settlement offers. They were all sent certified return receipt and i received all the green cards back on the last week of February. I am attaching a copy of the letter that I sent to the CA 's which I had copied off another message board. Would you please let me know if there is something in the letter I should have worded differently or not at all? I'm feeling a bit discouraged. I was hoping to have gotten somewhere with at least one of them. Thanks guys.

Sample letter:

Name of Collection Agency
Address of collection agency

Re: Collection Account for Original Creditor Account Number
Amount: $50.00

Dear Collection Officer,

This is for your information that the validity of the debt has been disputed. I am not aware of the account number and neither did you inform me regarding the existence of this account.

I am willing to pay this account IN FULL (or a settlement percentage, whichever is feasible) if you agree to immediately delete the account from the credit reporting agencies (namely Equifax, TransUnion and Experian) that you have reported to. My sole purpose is to get the item removed from my file. It should not be interpreted as recognition of the debt as valid or acknowledgment of liability for the same.

Incase you accept the terms of the agreement, the certified amount of $50 will be sent to the collection agency provided there is complete deletion of any reference to the debt from my file, from all the credit bureaus that you have reported to. As the full amount demanded will be paid back there should not be any waiting phase regarding the deletion of the items from the reporting bureaus.

Your agency should be deleting all information regarding the account from my credit files within 10 calendar days from the receipt of the payment as mentioned in the agreement. The terms of the agreement will not be discussed with anyone but your client on this account. No third party should be informed if contacted. No acknowledgment of debt or any kind of payment or settlement should be informed if contacted by the Reporting Agencies.

Following the acceptance of the agreement please prepare a letter on your company letter head unambiguously agreeing to the mentioned terms and conditions as the above settlement offer and have it signed by the authorized signatory of your agency. The letter will imply a legal contract, enforceable under my state laws.

In case I do not receive any approval letter within 15 days of your receipt of this letter I will withdraw this offer and request debt validation as per the Fair Debt Collection Practices Act.

Please communicate regarding this account to the address mentioned below.

Your Name
Your Address
State Zip Code.