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

Re: Telephone Exchange Usage in Low-Volume States

Neal McLain (
Thu, 18 Aug 2005 17:38:15 -0500

I wrote:

> 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.

Whereupon responded:

> I don't understand. Below is the local dialing plan we had when
> I was in school.

> Carbondale, IL, (Jackson County) 1971 General Telephone

> 618-453 - So. Il. Univ., Carbondale. IL
> 618-457 - Carbondale, IL
> 618-549 - Carbondale, IL
> 618-867 - De Soto, IL
> 618-684 - Murphysboro, IL
> 618-687 - Murphysboro, IL

> From/to any Carbondale NXX (1, 2, or 3): 5-digits allowed,
> 7-digits supported (618-453 required a ?9? to dial out from the
> university, but 5-digits allowed within the university
> PBX/Centrex/whatever)

> From Carbondale NXX (1, 2, 3) to De Soto (4): 7-digits required

> From Carbondale NXX (1, 2, 3) to Murphysboro, (5, 6): 7-digits
> required.

> From Murphysboro or De Soto to Carbondale, 7-digits required.

> (I believe that locally, only 5-digits were required in
> Murphysboro and only 4-digits in De Soto.)

My guess: the GTE Carbondale office used type "AR" (absorb repeatedly)
selectors on the 4th and 5th levels. Local (within Carbondale) calls
were dialable with only five digits, but if a local caller dialed all
seven digits, the prepended digits (4 and/or 5) were absorbed and
ignored. Nearby communities (7-digit dialable) were segregated on
separate levels (6th and 8th).

Which means that you could have dialed any combination of 4s and 5s
before dialing anything else, with no effect on the end result. For
example, you could have dialed 444554444555544443-XXXX to reach SIU.

With this in mind, the Carbondale dialing plan would have been:

----- ---------- ------------ ----------------------
----- ---------- ------------ ----------------------
1 11X Vertical service codes
2 Blank level ?
3 3-XXXX 618-453-XXXX SIU., Carbondale. IL
4 absorbed
5 absorbed
6 68X-XXXX 618-68X-XXXX Murphysboro, IL
7 7-XXXX 618-457-XXXX Carbondale, IL
8 867-XXXX 618-867-XXXX De Soto, IL
9 9-XXXX 618-549-XXXX Carbondale, IL
0 0 0 Operator
----- ---------- ------------ ----------------------

If my guess is correct, the 2nd level would have been blank (perhaps an
error tone). Did you ever dial it to see what happened? continued:

> Outside of these 3 exchanges, but within the 618 NPA:
> 1+7-digits required Outside the 618 NPA: 1+NPA+7-digits
> required

I suspect it's now:
Within the 618 NPA: 1+618+7-digits
Outside the 618 NPA: 1+NPA+7-digits

Unless the ICC has adopted the New York/California plan in 618, in which
case it's now:
Within the 618 NPA: 7-digits
Outside the 618 NPA: 1+NPA+7-digits

In this same thread, noted:

> As long as the dialing is unique, there is no reason that an ESS
> couldn't absorb digits just like an SxS could.

True. But it would have been enormously complicated, if not impossible,
to add more NNX prefixes as the community grew. A case in point: the
Carbondale office described above. (Note that I use NNX here, rather
than NXX, because most of the crossbar/ESS conversions occurred long
before 01/01/95.)

I realize that Illinois isn't one of the "low-volume states" that you
mentioned in your posting. But the problems associated with five-digit
dialing are universal, and occur in all states.

In 1971, there was only one unused level in Carbondale (2nd), so only
one new NNX would have been possible: NN2. The obvious choices would
have been 442, 452, 542, or 552 (to maintain the look and feel of the
existing NNXs); however, it appears that all four were already in use
NPA 618. So GTE would have had to find something on the 6th or 8th
levels (as it happens, 618-862 is still available today).

In any case, NN2 would have been the last non-conflicting NNX available
in Carbondale.

Now imagine yourself trying to play this game in every small community
across southern Illinois (or any other area code). In every case, you'd
have to pick a new NNX that:

- Wasn't already in use elsewhere in the area code.
- Didn't conflict with the local dialing plan.
- Didn't conflict with the dialing plan in any nearby community.

And you probably wouldn't even have Lotus 1-2-3 to help you do it!

Footnote: new prefixes in Carbondale today include 319, 351, 503, and
529. SIU's centrex has added 536.

Neal McLain

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