this number left a message stating they were from the IRS and threatening to take legal action if they weren't called back.

I received this same message from a robotic voice.  I am not calling it back as it reeks of phone scam.

The same number called me saying she is nikki Johnson.  I called the mumber back and was hung up on after to rings.  That was a threatening call.

received same strange female robot message from a nicki johnson of the irs threatening legal action.

http://www.irs.gov/uac/Tax-Scams-Consumer-Alerts
http://800notes.com/forum/ta-8c2f64bf2b91fa5/irs-warns-of-telephone-scam

Report the Calls to the real IRS here:
•report the incident to the Treasury Inspector General for Tax Administration at 800-366-4484.
•You can also file a complaint with the Federal Trade Commission at FTC.gov. Add "IRS Telephone Scam" to the comments in your complaint.

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Do consider getting a call blocking device or phone.. You can google them and many are around $50.00 or so.If its a cell phone, some already have call blocking, if not, there are free apps on the web that will work for some. You can google this. If you have a smart phone, there is a free app called call control. Iphones appear to have apps also. Another one is called Mr. Number.