What is the phone number for the IRS when you owe money for back taxes?

If this is about the income tax refund offset program through the FMS. You can use the below enclosed information and phone number.

The Department of Treasury's Financial Management Service (FMS), which issues IRS tax refunds, has been authorized by Congress to conduct the Treasury Offset Program. Through this program, your refund or overpayment may be reduced by FMS and offset to pay any past due child support, Federal agency non tax debts, or state income tax obligations.

Go to the IRS gov website and use the search box for Topic 203 - Failure to Pay Child Support, Federal Non Tax and State Income Tax Obligations

For additional information, FMS can be reached at 800-304-3107.