Ezekiel 33:3

If what he seeth the sword come upon the land
Or those that kill with the sword, as the Targum; as soon as he observes a body of armed men, more or less, marching towards the borders of the land with a manifest intention to enter and invade it: he blow the trumpet, and warn the people;
warn the people by blowing the trumpet, the signal agreed on; by which they would understand that an enemy was at hand, or danger near; or warn them by word of mouth, as well as by the trumpet, where he could do it, and when it was necessary.