Does iPhone automatically switch to EDGE?

Does iPhone automatically use edge once your disconnected from your wi-fi? if so, does it give you a warning or a choice to use edge? Because my plan is a basic plan. Calling only, no texting, or voice mail… or even caller id. Just basic calling, no unlimited data, or data at all. So I was wondering, will it switch to EDGE once my network closes down? Or i'm too far out of range from the router? Will it give me a choice to use EDGE? Same with 3g? Does anyone know?
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