Businesses and other organizations use telephone extension numbers to route calls within their internal telephone systems to the proper recipient. Typically, a caller dials the company's main number, then dials the extension for a connection to the person he wants to reach. Learn how to abbreviate an extension properly to make it clear what you mean when you write out an extension number.
Write or type the phone number in seven-digit format. Use hyphens to separate the area code, local prefix and four-digit line number. For example: XXX-XXX-XXXX, where the Xs stand in for the numbers.
Type a comma, followed by a space, after the phone number.
Type "ext." followed by a space and then the extension number. For example: XXX-XXX-XXXX, ext. 766.
Some people exchange the "ext." with an "X," so the extension reads: XXX-XXX-XXXX, X255. Style guides, however, prefer the use of "ext."