Roaming on your cell phone just means they charge you through the nose for calls made outside your 'home' area. You should be able to make calls from just about anywhere, especially along the Interstates, it will just cost you more. You should have plenty on your phone to get you through your trip, though it might be a good idea to keep the phone number for adding minutes on to it with you.
I have 150 units...I think that translates to about half that in minutes if I'm not home.
I just didn't know if I had to do anything special to tell the phone it's roaming. Guess not?