不通 is a versatile verb that describes something failing to be 'open' or 'flowing'. You use it for physical obstructions (a blocked road, a clogged pipe), failed communications (a phone that won't connect, a dead line), or abstract nonsense (an illogical argument, confused writing). It's built from the negative 不 + 通 ('open/clear'), and it functions as a predicate or a resultative complement.