A fax number is a unique set of digits that represents a telephone number connected to a fax machine. This number is used to send or receive documents via fax, a method of transmitting printed materials over telephone lines. Full definition
However, I could not find fax number on their website so I tried to contact them over chat to find if it is ok to fax Tax Return. (money.stackexchange.com)
To meet the demands of our clients and vet hospitals, we do use multiple fax numbers if issues arise with one of them. (consumersadvocate.org)
The evaluation process used available email addresses and fax numbers from the database. (academic.oup.com)