I had ATT which was bought by Cingular. When I moved here, there was no ATT/Cingular service available. Well there was an affiliated carrier. I got hit with MASSIVE roaming charges. The affiliate's users could use ATT/Cingular service without charge but not the other way around. Since it was a ATT affiliate my phone never thought it was roaming. They even took the and updated the address on the account, knowing that they didn't offer service here at the time. I killed the service (cingular moved in a couple months later).
Anyway...
When I called about all the charges, I could move to a "national plan" with no roaming. But was warned that in a couple of months since my calls would be on other networks over x% (don't remember the # now), they would send me a warning, and if it happened again (the next month since is was % of my calls) they would cancel my service AND still hit me with the termination fee. I cancelled. They tried to bill me for the termination fee and I've been disputing it ever since. (no service in area, no warning when roaming, etc.) They even offered to reinstate me after the came to the area... But they won't pay for the termination fee to Verizon so I told them to stick it.
Point -- just be aware that apparently Cingular has some policy out there to watch the % fo calls on other networks even when they say "no roaming".