Fax number format is the same as that of regular phone numbers. Each includes an area code, a three-digit prefix, and four remaining digits. International fax numbers look a bit different from domestic fax numbers, but they do resemble international phone numbers. The main differences in how fax numbers are written revolve around how they are formatted and punctuated.
Write the area code of the fax number. Place the area code in parentheses, though this is not necessary. Write a hyphen or period after the last number in the area code with no spaces before it if you choose not to use parentheses.
Write the three-digit prefix of the fax number. Include a space after the right parenthesis of the area code if you choose to use parentheses around the area code. Leave no spaces after the hyphen or period if you choose to write the number without parentheses.
Write a hyphen next to the three-digit prefix if you used parentheses around the area code or if you wrote a hyphen between the area code and the prefix. Write a period next to the three-digit prefix if you have used a period between the area code and the prefix.
Place the four remaining digits of the fax number next to the hyphen or period. Leave no spaces between the hyphen or period and the four remaining digits. A completed fax number might look like (xxx) xxx-xxxx, xxx-xxx-xxxx or xxx.xxx.xxxx.