The IRS has a plethora of fax numbers, with specific numbers to be used for specific purposes. There is no singular fax number that you can send any particular document.Consult the instructions for the form you want to file by fax for the number you should use. If you wish to respond by fax to a letter from the IRS, the letter should indicate the number to which you may fax it, if a fax response is acceptable.If you are still unable to determine the appropriate number to fax your filing, you can call IRS Customer Assistance at 1-800-829-1040 for individuals or 1-800-829-4933 for businesses to obtain additional assistance.