http://www.gsmworld.com/roaming/gsminfo/net_usbe.shtml

According to this website, Cingular Wireless in the United States uses
a GSM 1900 network. I'm looking to buy an unlocked cell phone that
operates on 900/1800/1900 GSM networks. I already have a SIM card with
AT&T Wireless, but because of the merger I am connecting via Cingular
GSM networks (at least the phone now says Cingular rather than AT&T).

1. Is there any risk that this unlocked cell phone will not work with
my SIM card and current plan?
2. Since I am currently an AT&T customer and have not upgraded to a
Cingular plan, am I actually still on AT&T GSM 850/1900 Networks (that
just force my phone to display Cingular) or am I actually on the
Cingular GSM 1900 Networks?

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Clarification of Question by craiganderson-ga on 13 Jun 2005 14:14 PDT

So as suggested, I called Cingular and was able to receive a partial
answer. They confirmed that the AT&T Networks no longer exist and that
as a current AT&T Customer I am currently on the Cingular networks
(which are the same networks that I would be on if I were to swap over
to Cingular by the way).

This has brought up a new question that they could not/would not answer for me:

1. What GSM Network are the majority of Cingular Networks. The website
I posted above indicates 1900 but the Cingular sales rep seemed to
think it was 850 as well.
2. Is there ANY way for me to tell what GSM network I am currently
using? Perhaps a device that detects wireless signals in the area?

Why not just call Cingular and ask them?  That why it is an official
answer, rather than someone's educated guess.  It will also be free.
1-866-CINGULAR

I guess my hope in posting this question was that I could get some
unbiased, truthful information. Cingular has every reason in the world
to either give me some spin or refuse to answer my questions. What
this boils down to, is that I'm talking about buying a cell phone from
someone other than them and avoiding the long-term contracts that go
along with that. As I said, they've got a few reasons to avoid my
questions.

Have you even tried calling Cingular?  I really doubt the customer
service rep is going to intentionally lie to you or refuse to answer
your questions.  They have always been very helpful to me in the past,
even in situations where it was not in their financial interest to do
so (e.g. Changing my plan to reduce my monthly cost, etc)

I called Cingular, but as expected they couldn't really give me the
answers that I was looking for. See my summary above, in an addendum
to my original question.

As far as a simcard is concerned, it is only a unique identifier for
the LINE of service you are on.  As long as your unlocked phone has
1900 GSM you will be fine.  I unlocked my own nokia and sony erisson
phones, it works all over the world on various different providers.  I
was able to actually register my own Tmobile sim card to another
provider abroad(although the simcard will tell the phone which band to
operate on ).

Cingular and AT&T merger was a good one, being that they are based off
of the same technologies, Cingular just turned AT&T towers into their
own.

I garuntee you will not have anyproblems with your unlocked phone.

ATT/Cingular most definitely make use of 800 (or "850" as referred to
in GSM systems) in addition to 1900.  Both companies are the
decendants of the old AT&T, and can trace their services back to when
800 was the only band available.

It's hard to say what portion of their markets use 800, 1900, or both
without checking all of them, but they definitely use both.

Calling the company is not nearly as good of an idea as it may seem. 
Customer service employees are wrong about these things all the time,
probably because it doesn't matter to the typical customer.

A phone that lacks either 800 or 1900 might work in your area, but
somewhere there will be an area it does not.

Try going to this web page and look at all of the current and recent
phones Cingular sells:
http://www.phonescoop.com/carriers/carrier.php?c=4

Notice they are virtually ALL listing 850 as well as 1900.

I agree with xcarlx.  ATT's signals are 850-1900 while Cingular's is
1900 mhz.  You should have either a dual band (850-1900) or tri-band
(850-1800-1900) phone.  If you have a 900-1900-1800 band phone, you
would have problems in some areas where ATT only operates their 850
mhz - Los Angeles being one of them.  I also own a Sony Ericsson T68i.
 And while commuting, connection would just "disappear" much like
going through some blacked-out zone.