The nature of the question implies that a “missions call” is something more specific than the general command given in the Great Commission. So in reply, I must say there are two possible answers. Broadly defined, a “missions call” is nothing more than fulfilling the task that has been given to every believer: to make disciples. But narrowly defined, and I think this is the point of the question, a “missions call” refers to a specific field of service. God calls all believers to the general task of disciple making and He calls some to a specific way of fulfilling that task. In general, we are to be missionaries wherever we are. Specifically, God may extend a special call to some people to relocate and perform a task that is out of the ordinary—such as relocating your family to a foreign country. In such a case, one should be certain that God has called him or her to that specific task.
There is a special call that comes to some, leading them to devote themselves full-time to the work of missions, either lifelong or for a specific period. Most other people are working to establish their lives and families somewhere, unconsciously giving material things most of their attention, and justifying their lifestyles with the words, “I don’t have a call to ministry.” It is a sad reality that it is almost impossible that such a person would ever hear a call. If the thought ever entered his mind he would dismiss it immediately. In the meantime, he falls far short of what he could be doing for missions. There is a shortage of people with a call, but there is a greater shortage of people with a heart in tune with the heart of God.
A missionary once told me, “Having a definite call to the mission field will hold you steady in times of doubt.” He had experienced misgivings and uncertainties resulting from stresses or apparent failures in his ministry. Knowing specifically that God had called him to serve in missions kept him from going home, he said. Pastors have testified similarly about their calls to ministry and even to specific churches.
However, there are successful, well-regarded workers who have served without such a specific calling. One, a “missionary kid,” has labored in missions all his life and says, “It just seemed like the natural thing to do.” Another felt responsible to fill a church’s pulpit until the congregation found a pastor and has spent most of his lifetime lovingly shepherding that flock.
Perhaps we could well say: Knowing that you are in the center of God’s will and determining to finish the job you are given are necessary elements of missionary service.
Since we are all called by God to be ministers of reconciliation in the world, we do not need any additional revelation from God to do that work. A “call” is subjective. Numerous missionaries have never had a “call” to missions, yet they have served God faithfully in missions. They saw the need and responded to it. Others have claimed to have a “call” to missions but never actually went to a mission field.
Just like some soldiers are given special orders, perhaps God gives some Christians “special assignments” (e.g. to serve in a particular place or capacity.) In the absence of special orders though, every soldier has general orders. The work of missions - reaching people who are not currently being reached with the gospel message - is sometimes a special assignment (Acts 13) and at other times it is part of general orders (i.e. loving God and loving people in the place where He has us at present.)
To correct a big misconception, not everyone is a missionary. Biblically, we are all witnesses. Thus, every Christian must find his/her role within the Great Commission – no exceptions. So, must one be called to be a missionary? From my perspective, one’s level of commitment to missions means more than if one declares himself to be "called." I know different people who are "called to missions" who are not currently on the field for one reason or another. I also know some people doing missions who are not specifically called to be missionaries, but have a deep commitment to and passion for mission work. Herbert Kane, a great missions teacher from yesteryear, stated that the phrase "missionary call" should never have been coined. I agree with him. Where is it in the Bible? Some would point to the “Macedonian Call.” No, that was provided for guidance and direction, as Paul was already a missionary. Think about it!
Why did Jesus ask His disciples to pray that "the Lord of the harvest would send forth laborers?" I believe it was because He foreknew the lukewarm and languishing state of His Church in the age to come – our age to be precise.
Why does one need a call when all have been commanded by Christ to make disciples of all nations? Does an authentic Christ-follower really need a "call" to keep him from muddling through a mediocre Christian life in our hedonistic and materialistic culture if he is gripped upon knowing, for instance, that there are 1.2 million Bedouins in Algeria who have never heard the gospel?
I gladly echo this sentiment of Jim Elliot: "Our young men are going into the professional fields because they don’t ‘feel called’ to the mission field. We don’t need a call; we need a kick in the pants."