This usually is a term used in a call to physical war. A call to arms was usually a bell or horn used to alert one to be prepared to fight. This is true in the spiritual also. Our weapons are not weapons of flesh and blood. They are weapons of prayer, worship and praise combined with the Word of God.
Prayer is the key element to the missions field.
Prayer acts in the supernatural realm. The realm that one cannot see. It is the most powerful weapon against the enemy of our souls. It is also to offer you the reader to become a vital part of missions through interceding for the nations of the world. The picture below shows Bishop and Sis.