What happens when a debt collection company ignores a cease and desist letter?

I made the mistake of taking out several payday loans and am currently attempting to get out of it. I have sent them all cease and desist letters in order to eliminate being called at work if I don’t answer my phone or call back within ten minutes of them calling me. One specific place is calling EVERYDAY as well as announcing who they are to co-workers that answer the phone. From what I’ve gathered their actions are not only harassment but also considered illegal once I have sent the cease and desist letter. I have also repeatedly spoken to them to stop calling me at work. What is the next step? This has got to stop. Now.