Since you have already mentioned the 'Cease and desist' action along with your validation letter and given them the option to be contacted by mail only after taking the debt settlement program, they should not have called your husband's cell phone number. How long ago have you sent the letter? If it is over 30 days and the account is appearing in your credit report marked as 'collections', you can show the validation copy to the credit bureau. Explain the company's failure of validating your debt. They will get this information verified through the company. If they fail in getting any info, the account will be removed from your file.
You can contact your lawyer and draft a notice of violation of the cease and desist letter. You have a case and sue them up to the extent of damages.