How to insert filter?

Please anyone help me out how to insert filters for the list?
I want the DNC numbers not to be dialed for the second time
For that i have to insert filter the leads...
I don't know how to insert filters.Please tell me the SQl command used in filter tab.
I want the DNC numbers not to be dialed for the second time
For that i have to insert filter the leads...
I don't know how to insert filters.Please tell me the SQl command used in filter tab.