I am playing as Ming and have sent a warning to Orissa and Ayutthaya. According to eu4wiki: "This will give you a call to arms if the target country attacks one of your neighbors. Country A cannot warn country B if B has already warned A. Lasts 20 years." However, Orissa and Ayutthaya seem to be at war with each other now and I did not receive a call to arms. Is this broken, or are there some conditions that the war must fulfill for the warning nation to receive call to arms?
You'll receive a call to arms only if warned neighbours initiate aggression themselves; that doesn't preclude them from participating on two sides of a conflict. Typically one of the warned countries is th aggressor against a third party not a neighbour to the country issuing the warning.
This strategy is of great value to human players, particularly when playing as a small country encroached by an aggressive, significantly larger nation. Typical example is all Balkan countries and the Ottomans in a 1444 start -- the AI playing the Turks often sends warning to all Orthodox countries in the region, and using allies is one of the few ways of getting stronger without incurring the wrath of the empire. One typically attacks an ally of their actual target so the Ottomans won't interfere.