通 (tōng) as a verb means 'to go through' or 'be connected' (e.g., a road or pipe). As an adjective, it describes writing or reasoning as 'coherent, logical'. As a classifier, it's used for phone calls, telegrams, or letters — unlike 封 (fēng), which counts sealed items like envelopes. 通 emphasizes the act of communication, not the container.