If you look at the sample CP05 that the IRS publishes on its website (https://www.irs.gov/pub/notices/cp05_english.pdf), you will see that all phone numbers have been redacted.
Why? Because the IRS reserves the right to change these numbers from year to year, and even region to region. I am not saying that they have, but they could. That is why they tell you the number to call on the upper right (which is redacted) - because it could vary between copies of the CP05.
I would call the general IRS number (800-829-1040) and ask them what fax number to use. Note that there can be wait times when using this number and it will get worse as we approach April 17th.
If you look at the sample CP05 that the IRS publishes on its website (https://www.irs.gov/pub/notices/cp05_english.pdf), you will see that all phone numbers have been redacted.
Why? Because the IRS reserves the right to change these numbers from year to year, and even region to region. I am not saying that they have, but they could. That is why they tell you the number to call on the upper right (which is redacted) - because it could vary between copies of the CP05.
I would call the general IRS number (800-829-1040) and ask them what fax number to use. Note that there can be wait times when using this number and it will get worse as we approach April 17th.