The caller left this message: "This is officially a final notice from IRS Internal Revenue Services. The reason for this call is to inform you that the IRS is filing a lawsuit against you. To get more information about this case file, please call immediately on our department number, 210-888-0979."
This call is very likely a scam. There are two immediate "red flags" to watch for.
The first red flag is the way they cleverly avoid saying they are calling "from the IRS." Instead, they say they are calling "from IRS Internal Revenue Services." Notice the plural word "services." This way, technically, they can protect themselves against any accusations that they are impersonating the IRS.
The second red flag is even more simple. If the IRS is truly going to sue someone, they are not going to place a phone call. Instead, they are going to send a certified letter.