If the IRS determines that an employee does not have enough withholding, the IRS notifies your employer to increase the amount of withholding tax by issuing a “lock-in” letter that specifies the maximum number of withholding allowances permitted for the employee.
Your employer will be required to take this action no sooner than 45 calendar days after the date of the lock-in letter.
As an employee - you may appeal that decision within 45 days.
You should receive a copy of the lock-in letter, and the IRS gives you some time before the lock-in rate is effective to submit a new Form W-4.
This form must include a statement that explains why you believe you’re entitled to a different withholding rate or number of withholding allowances.
You should send your Form W-4 and supporting documents to the IRS -
Internal Revenue Service Compliance Services Withholding Compliance Unit P.O. Box 9047, Stop 837 Andover, MA 01810-0
Once a lock-in rate is effective, an employer can not decrease withholding unless approved by the IRS.
You won’t be able to decrease your withholding unless the IRS approves it.