A fax number is a phone number that can be used to contact a fax machine. It is simply a landline telephone number used for regular calls, and by plugging the fax machine into that standard landline, you are using the phone number allocated to that landline as a unique fax number. The length of fax numbers varies from country to country, but in the United States, it is usually 10 digits. The format of a fax number is similar to that of a regular phone number, with area codes and subscriber numbers. There are three main fax number formats, depending on whether you send documents locally, regionally, or internationally. Many premium online service providers offer an online number, which lets you send and receive files using your smartphone or computer. Some fax machines can have a phone line for testing connectivity and making phone calls, which means that if the recipient answers the phone, it works like a regular phone line.