Someone who was working in the tower must've put their phone on plane mode.
Twin Tower Airplane Mode
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Explanation
This is a dark joke referencing the 9/11 attack on the Twin Towers. It suggests someone inside the tower put their phone on airplane mode, which is a common thing to do during a flight. The humor comes from the absurd and tragic situation, and the implication that the plane mode was activated when the planes crashed into the towers.