Just got the same message from Nikki Johnson from the IRS attempted call back my True Caller app showed the number to be a mobile phone unregistered in California. Sounds like a complete scam.
I got the same message today and had to listen to the call three times to make sure I was hearing correctly. "Officer Nicki Johnson of Internal Revenue Service"...not 'THE Internal Revenue Service'. When I called the number I got an advertisement for Magic Jack and then a statement that the Magic Jack number I called has not been assigned to a customer.
Darn is must suck to try to run a scam and then use the wrong number!!!