Anytime a government agency needs to contact you they use the postal service because it supports another government division. They will never call you about money you owe. You will receive a certified letter. So all situations like this are scammers.
Even if you are on the DNC list, this list is very difficult to enforce as the perpetrators use a computer or auto dialer to call. The DNC list is only for direct person dialing and does not apply to business numbers. That's how the perps get around it! Get real people!! If you don't recognize the number don't answer it!! Make that decision and there will be a lot less stress in your life.