Answerers @ Google,

This will be my first question to Google Answers, so I'm starting
small.

Issue:

I control a number of different LDAP systems and would like to codify
contact information.  Specifically, I am dealing with phone number
extensions.  Almost all of my contacts have specific ten digit phone
numbers, but a few are now beginning to show signs of using a main
number with a 3-5 digit extension.  My first thought was to use
555-515-1200 221 or 555-515-1200 x221, but these are just my own
preferences.  The LDAP data stores are all U.S. based and therefore
the ten digit 555-555-5555 is the convention I chose a long time ago,
again this was just a preference.

Question:

Since this is a $2.00 question, I suspect my answer will be in the
form of links.  I would like some links to either U.S. (preferred) or
International standards of listing extensions to a phone number.  I am
not sure these types of standards exist, but I would suspect the U.S.
government would have at least codified how they would like phone
numbers to be displayed.

Answer:

Any listing which provides information on how best to display phone
extensions.

Or

A link which indicates that no standards exist and that implementers
are free to choose from a number of different options.

Thank you,

Black

Two well-respected reference works, the AP Stylebook and the Gregg
Reference Manual, agree that "ext." is the preferred way to indicate a
telephone extension:

"telephone numbers 
Use figures. The forms: (212) 621-1500, 621-1500, MU2-1500. The
parentheses around the area code are based on a format that telephone
companies have agreed upon for domestic and international
communications. For international numbers, use the parentheses around
the country code and the city code (where required): (44-20)
7353-1515. Use hyphens, not periods. The form for toll-free numbers:
(800) 111-1000.

If extension numbers are given: ext. 2, ext. 364, ext. 4071. Use a
comma to separate the main number from the extension."

AP Stylebook
http://webreg.tccd.edu/Datatel/openweb/TCCMenuItems/Employee/APStylebook.pdf

"The Gregg Reference Manual recommends this format for phone numbers:
(708) 532-1900, Ext. 2804 (but write out Extension in formal
correspondence)... Authority: The Gregg Reference Manual by William A.
Sabin. 9th Edition. McGraw-Hill: New York. 2001."

The Grammar Logs
http://webster.commnet.edu/grammar/grammarlogs4/grammarlogs530.htm

Most of the university style guides online follow this convention, as
well:

"Area codes should be placed in parentheses, followed by a space.
Example: (909) 607-2000. The word extension should be spelled out, if
possible. If there is insufficient space, the correct abbreviation is
"ext." Extensions should never be preceded by an "X."

Pomona College Style Manual 
http://www.pomona.edu/publicaffairs/styleguide.shtml

"telephone numbers

 Preferred: 202/555-4832
 Acceptable: (202) 555-4832
 Unacceptable: 202/555/4832

 Extensions: 202/555-4832 ext. 123"

Virginia Tech Editor's Style Guide
http://www.unirel.vt.edu/style/styleguide.html

"telephone numbers -- If a publication is strictly for on-campus use,
omit area code: 392-0186; if publication may or will be sent off
campus, include area code in parentheses with a space between
parenthesis and number: (352) 392-0186; if including more than one
extension, use a solidus (/) between the numbers: (352) 392-0186/0188.
When listing an extension within an office, write the number followed
by a comma, a space, "ext." and the number:  (352) 392-0186, ext.
132."

University of Florida Communications Network Writing Stylebook
http://www.pr.ufl.edu/w_style/s-z.htm#telephone%20numbers

For many years I worked in a government office. We were instructed to
use "ext" (without the period), and were specifically cautioned
against using "X" or "x" to indicate an extension. The reason we were
given is that a caller could misunderstand the "X," and might press
the number key which bears the letter "X", thereby inserting an
unwanted "9" into the telephone number, which would cause a failure to
connect.

Search strategy:

Google Web Search: "phone OR telephone numbers with extensions"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22phone+OR+telephone+numbers+with+extensions

Google Web Search: "phone OR telephone numbers" extensions "style
guide"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22phone+OR+telephone+numbers%22+extensions+%22style+guide%22

I hope this is helpful. If anything is unclear, or if a link does not
function, please request clarification; I'll be glad to offer further
assistance before you rate my answer.

Best regards,
pinkfreud

For a mere $2.00, I have been provided an answer that I would be proud
to present to my management team as reasoning for my recommendation. 
I was also impressed with the readability and fullness of the answer.

Thank you pinkfreud

I would have to disagree with the assertion that (44-20) 7353-1515 is
the correct way to present an international number. Parentheses are
used round an area code to indicate it may be optional for callers
dialling from within the same area. In international dialling,
parentheses must not be used as the entire number must be dialled. The
suggested format (44-20) might be read as suggesting a company's
*internal network* dialling code, rather than a PSTN international
number.

The international convention is to show the country code (for the UK =
44) and number prefixed by "+". The "+" indicates that callers should
dial the appropriate International access code according the country
from which they are calling.
 UK Telecom FAQ
 http://www.gbnet.net/net/uk-telecom/p2-2.html

Incidentally, it is never correct to have both a plus sign and
parentheses in a telephone number. The plus sign indicates that you are
using the international format, which expressly forbids parentheses.
See ITU-T [International Telecommunications Union] Recommendation
E.123 for more information
  Linc Madison writing in news:uk.telecom
  <240420011929193276%lincmad001@telecom-digest.zzn.com>

Therefore, if the directory is to be used internationally, the +
format should be used as it can always be converted to show the actual
number required from the user's location to reach the distant end. I
don't know if there is an ITU standard for showing extension numbers.

Owain

Thank you very much for the five-star rating and the generous tip!

~pinkfreud

This answer has been closed, but I thought I would note that Microsoft
Outlook 2003 now allows for extensions to be added to a contact item's
phone number and that the format takes the form of +1 (555) 555-5555 x
321.  So we can see that Microsoft does not seem to be following AP
Stylebook or Gregg Reference Manual and is going against the
recommendation of Pomona College.

Thank you,

Black

I would bet that Microsoft will be influential in setting a new
standard. Anyone who is criticized for using 'x' to represent
'extension' can now point to a mighty powerful precedent-setter. ;-)

Personally, I think the use of 'x' or 'X' is widely understood, and
the notion that someone might think this meant the letter 'X' on the
phone keypad is outmoded.