This appears to be a scam of some kind.  Preying on people who think they might have a tax problem.  IT IS NOT THE IRS.  It is a scamming company.  If you google this phone number, you'll see it's been reported all over the place.  Do not answer.  And if you get mail from them, it's also a scam.

One word " SCAM "

The name of the Company is National Tax. They are Federal Tax Attorneys. They can not guaranty the settlement offer they propose.

I have been told by the IRS that this is a SCAm. The agency related to this phone # is trying to get you to pay a settlement amount for back taxes. These people are not associated with the IRS. THIS IS A SCAM!!!!