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.
