A fax number is a unique telephone number assigned to a fax machine, allowing it to send and receive documents electronically over a telephone line. When a document is sent via fax, it is scanned and converted into a digital signal, which is then transmitted to the recipient's fax machine using their fax number. Using a fax number is common in business settings, where documents like contracts and forms need to be shared quickly and securely. Unlike email, which requires internet access, faxing can be done over traditional phone lines, making it a reliable option for many organizations.