Franklin Collection Services Calls multiple times a week on my work number but leaves no message Tupelo Mississippi

This has been going on for at least a month. The first time, I picked it up but did not receive a response. They called later that week and I ignored it. My work number is on the National Do Not Call registry yet they keep calling once or twice a week. 

I don't know why they are calling. I have never received anything but dead air when I answer the phone. I was only able to get the company name by looking up the phone number. I have complained to the FTC about this and I have seen that it is possible to sue this company for violating purposes. I'd like to do that if possible since I have absolutely no idea why they are calling.