Cawley & Bergmann keep calling my workplace after I told them to stop. What can I do?

I'm at my wit's end with a collection agency called Cawley & Bergmann. They started calling my cell phone constantly, so I specifically asked them to stop contacting me at work. But they keep calling my office line anyway, and it’s getting really awkward with my boss and coworkers.

They’re also extremely aggressive on the phone, making threats about legal action. I feel like they are crossing a major line. I know I have some rights under the FDCPA, but I'm not sure how to actually enforce them.

I came across this page that explains what they're doing might be illegal, but I need practical advice.

How do I make them stop calling my job? Should I send a letter? Any help or similar stories would be really appreciated.