TELECOM Digest OnLine - Sorted: Re: Telephone Exchange Usage in Low-Volume States

Re: Telephone Exchange Usage in Low-Volume States

Neal McLain (
Wed, 17 Aug 2005 07:38:50 -0500 wrote:

> In many places in the U.S. the demand for telephone exchanges is very
> high for a variety reasons. This has result in area code splits and
> overlays. NJ started off with one area code and now has nine.

> But some states still only have one area code. I understand some
> states are not growing very fast in population, indeed, some rural
> towns are losing population. This includes: Alaska, Idaho,
> Montanna, North Dakota, South Dakota, and Wyoming. (Not counting
> some other single-code states).....

I assume that by "exchange," Lisa is referring to an NXX code.

> Given the rural/low growth aspect of places in some of these states,
> I was wondering if telephone service may still have some old
> fashioned features to it. For example, would such areas have:

> 2) Five digit dialing in some areas not well populated or served by
> community dial offices?


The misconception here is that, after the introduction of DDD, telcos
retained 4- or 5-digit local dialing as a convenience to users. Telcos
retained 4- and 5-digit numbering only as a temporary stopgap measure
during the transition from SxS to crossbar or ESS.

In order to accommodate inbound DDD, it was essential that every
number have a 7-digit format. But SxS switches couldn't accommodate
7-digit dialing, so telcos faked 7-digit numbers by prepending dummy
digits. Local calls continued to be dialable with only four or five
digits; however, if a local caller actually dialed all seven digits,
the prepended digits were absorbed by "absorbing selectors" --
i.e. ignored.

This situation led to numerous conflicts between local numbers
(dialable as 4 or 5 digits) and non-toll calls to nearby communities
(dialable as 7 digits). To avoid such conflicts, telcos had to devise
special dialing plans. AT&T documents dating from 1975 describe such
dialing plans in detail ["Typical Trunking Diagrams for Step-by-Step
Offices." "Notes on Distance Dialing." AT&T Engineering and Network
Services Department, Systems Planning Section, 1975. Appendix A,
Section 4].

The legacy of these old dialing plans can still be seen today in the
numbering assignments of many outlying communities surrounding
mid-sized cities, even though the old SxS switches have long since
been replaced with ESS. In these communities, most/all of the NXX
codes assigned to the outlying communities start with a numeral that
was not used in the central city in the SxS days. Two examples that
come to mind:

-- In Ann Arbor Michigan, local numbers were a combination of
four- and five-digit numbers, all served from the same
central office:

2-XXXX Huron office
3-XXXX Huron office
4NX-XXXX Outlying communities
5-XXXX Huron office
6XXX Huron office
7XXX Huron office
8XXX Huron office
9XXX Huron office

The 4NX codes are still in use today, even though one
of them (South Lyon 437) is now in a different area
code (248).

-- In Madison Wisconsin, all local numbers were five digit,
served from four central offices:

2-XXXX Pflaum office
3-XXXX Sylvan office
4-XXXX Kedzie office
5-XXXX Main office
6-XXXX Main office
7-XXXX Main office
8NX-XXXX Outlying communities
9-XXXX Kedzie office

The 8NX codes are still in use today.

A third example -- Centerville, Iowa -- was cited by Mark Roberts in a
posting here on TD a couple years ago. As I noted at the time, even
though Centerville's old SxS switch retained 5-digit dialing, new NNX
codes, even within Centerville itself, would require 7-digit dialing.

Neal McLain

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