Your fax number is the number of the phone line connected to the fax.
With a fax machine or computer running fax software, the fax number is the phone number of the attached phone line. Calling the toll-free number 1-800-444-4444 (operated by MCI) will tell you the line’s phone number.
With an online fax service, the fax number is the number assigned by the fax provider. Log in to the service and check settings to see the fax number assigned to your account.