TELECOM Digest OnLine - Sorted: Overlay NPA-NXX Codes (re: Caution: Unidentified Callers)

Overlay NPA-NXX Codes (re: Caution: Unidentified Callers)

Anthony Bellanga (
Sat, 09 Sep 2006 20:49:18 -0600

PAT - DO NOT display my email address anywhere in this post! Thanks.

Paul Robinson <> wrote:

> 303 and 720 are overlay area codes for the same place, the Denver
> Colorado metropolitan area. No telephone company would assign as
> prefixes, codes which are used in the same or an adjacent area
> simply because the possibility of confusion would be tremendous.

> I live in Northern Virginia, which has both 703 and 571. Nearby
> is Washington, DC (202), South and West Maryland (301 and 240),
> and south virginia (540). There is not going to be a 703-571, a
> 571-703, a 202-703, a 202-301 or any similar combination because
> of the potential for confusion.


Right now, there happens to be no assigned 303-720 nor 720-303 office
codes. But read on further down -- I will repost what I originally
said back in mid-July when this thread originally
appeared. Neustar-NANPA, the assignment body for US-based NPA-NXX
office codes does NOT flag either 303-720 nor 720-303 as "NOT to be
assigned" in their web-based databased. Afterall, this is an overlay
situation, and since ALL TEN DIGITS MUST be dialed, in the long run,
there is no conflict or ambiguity with such assignments. And inn some
overlay areas, the same digit combination is used for NPA and an NXX
within that NPA!

You also might want to go through some of the entries in NANPA's
NPA-NXX assignment database. In some cases, there are indeed flaggings
of 'UA' which means unassignable -- i.e., NOT to be assigned. And this
is still the case with certain combinations in overlay areas. But that
doesn't mean that in the next ten years or so, when ten-digit dialing
and overlays becomes more the norm throughout the US and Canada that
such combinations won't still be flagged as not assignable. And as I
mentioned, there are such assignment situations in some places

Even in places where there aren't yet overlays or mandatory ten-digit
dialing, there are some cases where the numerics of an adjacent NPA
code, even one which is adjacent with local calling arrangments --
those numerics can also be used as an NXX office code in other
adjacent local NPA codes. The ambiguity is usually eliminated by the
use of a mandatory 1+ before ALL ten-digit calls, regardless of
whether the ten-digit number is local or toll. And all "home" NPA
calls in such area codes is still permissive as "just" seven-digits,
sometimes without differentiation as to local vs. toll for that
seven-digit "home" NPA call.

And here is the original post and my reply in this thread when it
originally appeared back in mid-July:

DLR <> wrote:

> I got a call yesterday on my Sprint cell phone from 303-720-1234.
> I had no idea at the time where area code 303 was so I let it go
> to voice mail. But they didn't leave a message. So today I tried
> to call it back and got a "no such number exists" type of message.
> And I can't find anyone with this number via Google searches and
> don't really want to pay for more information. Now to be honest it
> looks a bit bogus to me. 1234 and all.

It is bogus. 303 is the area code for the Denver CO Metro area. (At
one time, the entire state of Colorado used 303, but that was some
time ago). 720 is being used as a central office code within the 303
area code in your situation, but the 720 numerics as an area code also
happens to be the overlay area code for the Denver CO Metro area.
Since ten-digit local dialing is mandatory within the Denver Co Metro
area of area codes 30 303 and 720 in overlay, there isn't anything
"wr-ng" with having a 303-720 code, nor a 720-303 code, nor even a
303-303 or 720-720 code. Ten-digit local dialing is mandatory in
overlay areas.

However, it happens that at this time, there is no such 303-720 code
assigned in the Denver CO Metro area. I looked up the central office
code reports at NANPA's website, and 303-720 is
unassigned but still available for assignment.

So, you probably got a call from a telemarketer sending bogus
caller-ID information. Typical. :(

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