the IRS has many fax bnumbers probably thousands. so if you have a document with a phone number call it to get the correct fax #. if nothing then you'll have to contact the IRS 800-829-1040
First choose your language.
You should then be transferred to an agent, but there is likely to be a long wait.
Be careful. This is a public Internet forum. Don't go by a FAX number provided in a public user community forum like this to send such sensitive documents. Someone could even try to scam you into FAXing it to them instead of the IRS to steal your info. Or someone might tell you to send it to the wrong IRS department.
Did you receive an IRS letter? If so, it may have the FAX number to use or the number to phone for info.