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In the absence of an election, a corporation is a C-Corporation. To form an S-Corporation, you would have filed this form at some point:
A notice would have been sent from the IRS stating your S-Corporation status acceptance.
You may contact the IRS with your EIN number and ask what kind of corporation you are. The IRS would know, and they would mail you a notice, not call you. This actually sounds like a scam.
Nevertheless, corporations are required to file returns, even without income, so if you haven't filed one you need to. File a final return to relieve yourself of this requirement in the last year you conducted business (without having conducted any business, this would be the year you created the corporation). File a $0 return with the final return box on page 1 checked (Form 1120).
(2009 form as example)
Contact information for the IRS can be found here:
How do I find out if my company is an "S" corporation or a "C" corporation?