I've gotten two of these stupid calls this morning, and they leave a garbled message about calling the IRS back. SCAM!!! The IRS does not call people like this, they will bill you through mail or email first.
I am getting these robocalls almost daily now from different area codes and states but always the same damn message that the IRS is filing a lawsuit. If I ever find the robot making these calls I will personally pluck each and every transistor one by one until it dies a glorious electrical death.