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IRS website for a CP14 notice - https://www.irs.gov/individuals/understanding-your-cp14-notice

What should I do if I disagree with the notice?

Call the phone number located in the IRS Help section of your notice. Please have your paperwork (such as cancelled checks, amended return, etc.) ready when you call.

If you used the IRS payment website to pay the taxes owed, then you will have a Confirmation Number from the IRS for that payment - https://www.irs.gov/payments