I worked as a debt collector a few years back and it is not true for most of the states in the US. A SIGNED & WRITTEN cease and desist letter needs to be sent by certified mail to the company for them to stop calling you. They will continue to call you. I suggest you pick up the first time, say yes its me, then hang up. once contact is made they are not allowed to call you again that day. As far as them being threatening, if it's for unsecured debt like credit cards or school loans, you can document it and probably take it to court. If it is for secured debt like a mortgage or a car, it is not a threat, they are trying to keep you paying for your car or house and its in your best interest to listen up or get repo'd or foreclosed on.
These people call to harass me about sending a payment a WEEK BEFORE payment was even due. I was told that my payment from the previous month was late and that they needed $60 up front over the phone NOW. When I asked if this $60 was included in my minimum payment due the next week, the guy on the phone said "yes." My next question was "THEN WHY ARE WE HAVING THIS CONVERSATION?? You'll get your payment next week."
they called me a day before pament was due, left no message the first 3 times they called....finally picked up to a guy with an indian acent wanted the full payment 2 days after it was due, told them to call the usps cause the check was in the mail