You received this notice because the IRS found a discrepancy between the amount of estimated payments you reported on your return and the amount of payments that they received for your account. This notice may also show changes the IRS made while processing your return.
The first steps to take are to verify that all your estimated tax payments are listed and then to check the amount that has been applied from a prior year (if applicable).
If you come to agree that the notice is correct, you will need to pay the amount due on or before the due date that is listed on the notice. If you cannot make pay the full amount, you can make payments. You may also want to correct the copy of the return you have and save it for your records.
If you do not agree with the changes, you will need to contact the IRS within 60 days from the date on the notice. You can contact them by phone by calling the number on your notice or by mail. If you choose to contact the IRS by mail, include a copy of the notice and allow 30-60 for a response or resolution.
For more information regarding your CP23 notice, please visit the IRS website.