I think the following should explain it. But if
I'm wrong, I'm sure plenty of people will correct me.

About five years ago Cingular had a physical network presence
in California, and t_mobile had theirs in NY. The two companies
made a handshake to allow each others customers to invisibly
cross-roam when in teh other's area.

In other words, if you were a cingular customer in NYC you'd
be on t-mobile (and vice versa).

The base station in NYC, though, would be putting out
a numerical ID that's associated with t-mobile.

So... if you had a multi band international phone you'd
either see the numerical ID or you'd see "t--mobile", depending
on whether the SIM "knew" the translation.

But... if you purchased a Cingular phone in the NYC
area, it would show "cingular", even though it was
using the t-mobile towers.

Same thing, in reverse, for California. A multi band
international phone over there would either give the
numerical ID or the Cingular name.

This would even be the case, *sometimes*, if you had
a t-mobile phone in Calif.

If the phone was specifically marketed by t-mobile in
California, they'd have programmed the SIM to translate
that Cingular ID number into the name "t-mobile".

But.... other phones wouldn't have that, so they'd
still show Cingular.

Now you've got a t-mobile branded and California
marketed phone, so it's seeing the (cingular) base
ID and is displaying it as t-mobile. The other phones
show the, for want of a better term, cleaner interpretation.

NOTE that this handshake arrangement is being phased
as each company builds out its own network, but
O don't know thje schedule.

T-Mobile in California uses the cingular network (still) though at
some future time cingular will be migrating to the old AT&T Wireless
network (since cingular bought AT&T Wireless) and cingular sold the
old network to T-Mobile and T-Mobile likely has not changed the
operator identification yet (likely because cingular is still
transitioning to the new network and has agreed with T-Mobile that
they can use their "legacy" network for three years if necessary.)
Bottom line is unless you have a recent EONS T-Mobile SIM you will
only see two networks cingular (310-17) and AT&T Wireless (now
cingular) (310-38.)  In California and Nevada T-Mobile shared network
with cingular so if you have either an old VoiceStream SIM (or even
some T-Mobile SIMs) it will say cingular even if you're a T-Mobile
subscriber.
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The SIM card controls this;  also the internal phone locking, if any.