Just got the same call today. Did not answer, Message left say this was final attempt ( I hope so) and to call them back at another number which I did not. I did however, go on the IRS.gov. website and reported the phone numbers.
Left recorded message stating they were from the IRS and that they had tried to contact me many times so that this was their final attempt and that they were now going to file a law suit to recover back unpaid taxes. The caller id number did not match the number I was to call back to speak to a representative. When I did try the number it was always either busy or it rang and rang and rang for a long time until the answering machine finally stated that I could not leave a message at that time and for me to try to call back at a later time.
Should I also contact my district attorney here in my county to report this and get these criminals prosecuted?