Most of these calls are coming from India and Pakistan.

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. This is your best bet to stop the calls!

203-716-8366 scammer. Called me at 11:56am says serious call back to day or go to jail.

This man called me today claiming Adam Foster from the IRS and when I called him back it was clear he was not from the IRS and I could hear dogs barking in the background. If this man leaves you a message don't call him back!

I received a call from this number too, claiming to be the IRS agent...God blessed me too....

Caller named Alan Foster with either Indian or surrounding area accent but good command of the English language telephoned to let me know I was a person of interest in a IRS investigation against me, that I needed to accept papers from their investigators that are coming to the home or they will involve the Sheriffs Department. I have received no mail from the IRS and the local office said they are unaware of any investigation. Advised me of IRS scam and to ignore calls.
The caller was identified as Alan Foster

Read more at https://www.callercenter.com/203-716-8366.html#56zRKBajvt4o0lSY.99