No, buy a plan is better, unless you have land phone at home.

In US, all cell phones including long distnace in US (so you can call anywhere in US). Roaming is usually for out of the carriers' service area, and it "borrows" other carriers' network. Many cell phone has an option to warn you if the call is roaming.

Dial your phone card's access number is a normal call. But be sure to turn your cell phone service's international off. It tends to have some problems there sometimes.