I have an AT&T national plan.  My understanding is I can
call from anywhere to anywhere in the US without accruing
roaming charges as long as it says "AT&T" in the phone's
display.  If it says anything else it's going to cost.  I've
always assumed that "Extended Area" in the display meant
roaming charges, but I never checked that out with AT&T.

Unfortunately you can't always depend on the indicator on the phone to
know whether you'll have to pay roaming fees.  For example, in
Connecticut there's a special arrangement for customers on both the
local and national TDMA plans for free (no charge) roaming on Cingular
(Extended Area) and Verizon (roam in analog).

But in many other places, you will be charged roaming for using
another carrier's network (when the display doesn't say AT&T).

Ultimately the decision about whether it's roaming or not comes down
to the billing computer and whose tower you're using.  And that's
pretty difficult to tell sometimes if it's included or not.  Some of
the AT&T local plans used to include a lot of roaming but then other
plans don't so there's no general rule other than if the display
doesn't say AT&T then you might be charged.

Lest you think this is unique to AT&T, Verizon customers have similar
complaints about the roaming indicators, especially on the America's
Choice plans.  In a similar fashion it's the billing system that
determines roaming fees, not the display on the phone.

I don't believe "Extended Area" means Roaming, otherwise I would be paying
$100's extra every month.  I have the digital one rate plan, and spend lots
of time in "extended areas".  No extra charge.  I don't know how other plans
work.

"Extended area" means roaming on a preferred carrier, usually in
Digital.  So yes, your phone was trying to tell you something
afterall=)

As for your plan, you will have to look for the specific coverage it
covers and see where you were.  Typically, unless it's a situation of
your being right on the border, the system bills out correctly.