If you're on a plan called "free and clear," long distance is included in your plan at no extra charge while you're anywhere on the Sprint PCS network (the phone does not say "roaming").
Look at your bill. If it doesn't say "free and clear" anywhere on it, call customer service (as if they're worthy af that title, bleh!), wait on hold (and wait, and wait, and wait...), and ask them how your specific plan works.