The point is that the hash is effectively reversible at that scale. 10 numeric digits is 10 billion possibilities. It's actually far less than that because area codes don't span the entire 3-digit number range, and most area codes are not heavily populated. But even so, creating a lookup table by computing 10 billion hashes is today a comparatively easy task, even for expensive hash algorithms. It's a matter of only a few seconds to minutes work with relatively common hardware (high performance GPUs).
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The point is that the hash is effectively reversible at that scale. 10 numeric digits is 10 billion possibilities. It's actually far less than that because area codes don't span the entire 3-digit number range, and most area codes are not heavily populated. But even so, creating a lookup table by computing 10 billion hashes is today a comparatively easy task, even for expensive hash algorithms. It's a matter of only a few seconds to minutes work with relatively common hardware (high performance GPUs).