From an article in our June 2018 issue of the California Taxletter:
Form FTB 3904
If the FTB suspects identity theft, they will send Form FTB 3904 before processing the return. To resolve this issue, the taxpayer must call the FTB. You may not mail or fax a response.
While you may call on behalf of your client to resolve the Form FTB 3904, the taxpayer will need to call the FTB to answer security questions that you will not likely be able to answer, such as where they have had bank accounts and prior addresses. The notice states that the taxpayer must call within 30 days or the FTB may not be able to process the tax return or issue a refund.