"...we paid both the quarterly estimated taxes and the amount due as calculated by TurboTax..."
A CP23 letter usually means that the estimated tax amounts you entered on your tax return did not match what the IRS had on file for your account.
https://www.irs.gov/individuals/understanding-your-cp23-notice
TurboTax guarantees that its calculations will be correct, but it can't guarantee that all user entries will be correct.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
