National XXXX keeps calling and leaving messages. I spoke with their President at one time months ago and he promised to remove my number, but they called again several times lately. 
This is the XXXX time I have had to write you a letter. This is a copy of the letter I sent today. 

You keep calling my office number and leaving a message for a ( sounds like ) XXXX XXXX ... YOU MIST TAKE MY NUMBER OFF YOUR LIST. Months ago, I had to go all the way to the head person. He promised to take my name and number off the list. If I have to do this again, I will file a complaint and make sure I gather all my emails and call info. The harassing messages are coming from a XXXX call center -- - number XXXX XXXX. 

Besides, I understand it is not legal to leave a message about a debt anyway! 
DO NOT CALL my numbers again!