Use the figures only, without the word telephone preceding them. Area code is not enclosed in parentheses but is followed by a hyphen.
An exception to the previous rule is when both the telephone number and the fax number are given. Use the following format for such instances.
In all internal communications, use the five digits of the phone number with a hyphen. Do not use the word extension or the abbreviation ext. in telephone references. In external communications always include the area code.