I was looking at our wireless account statement on the AT&T wireless account today, and I saw an anomaly: Shared Mobile-to-Mobile usage was zero on one of our two lines. Like most people with shared plans, we use our phones mostly to call each other, and most our friends are also on AT&T. So Mobile-to-Mobile minutes are the ones we typically use the most. Our usage pattern, our minute plan, our phones, our friends' providers, nothing changed since last month, so why are we not using our much cheaper Mobile-to-Mobile minutes as we should?

Here is the answer:

Q: How do I know whether a call will be counted as part of my unlimited Mobile-to-Mobile calls?
A: With GSM America National, your unlimited Mobile-to-Mobile minutes are available for calls placed to or received from the applicable Mobile-to-Mobile Service Area to or from another AT&T Wireless customer when both are on the portion of our domestic network that we own and operate and "AT&T" or "AT&T Wireless" is displayed on the phone.
Pasted from http://www.attwireless.com/gsmamerica/gsmamerica.jhtml

Our phones were displaying “Cingular” every now and then, and we thought the merger was coming through, we were getting better coverage, it was cool! But the whole time we were being overcharged.

In fairness to AT&T & Cingular, pulling off a a merger like the one they are going through is a tough engineering task. The customer support person indicated that they would go through our bill at the end of the month to see if any refund is appropriate. It would be a good idea to pay attention to this as you look at your next bill.