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For a salaried professional, answering a telephone call from work could be considered to put one "on firm business". Since the firm can call at any moment, 24/7, that could put one in technical violation of the policy even while at home, or at the range for that matter. Not answering the phone would likely also put one in violation, and one's job in jeopardy.

It's not illegal, but it would be tough to enforce under those circumstances.