IRS Scam! DO NOT answer and DO NOT call these people back! It is a full on scam. If you do answer, DO NOT give them any information! IRS does not call you.
I have received daily voicemails from this number telling me that (a) there is a warrant for my arrest (b) they are the IRS. Total scam. Do not call the number back (you might be charged just for calling the number). Do not give them any information. If possible, block the number and/or put it on a spam list to notify your cell phone provider.