Credit card debt from 1998 not ours?

Don't worry about sending letters. If they continue to call after you send the cease and desist letter, the next time they call, let them talk for about 2 minutes, then just tell them this calling is being recorded on the request of your phone co. If they still call, then call your phone co and tell them to put a tap on your phone. Once that happens, they'll have bigger problems to deal with.