That’s really weird. I am getting exactly the same results using that patch body. The company’s phone or fax are still empty after that request. I tried using the other values for field as well, “FAX_NUMBER_FIELD_UNSPECIFIED”, “FAX1”, “FAX2”. They all left the Fax number as “null”. The patch does receive a success status code, it just doesn’t update the numbers.
I even tried creating a new Company with no phone/fax and adding to that, still no luck.
Edit: Additionally, if I add a number to the company through the web interface, the exact Patch request in my original post works to change the number, just not to add it if it’s not there.