Are people ever actually 'called' to go to the mission field? Now there were in the Bible when men and women were spoken to specifically and personally to embark upon a special mission. Whether it was to fight a certain enemy of Israel or call the nation to repentance, some individuals were given a special commission to serve the Lord in a particular field of ministry for a particular time and function. But is that the case now? Those individual played a special role in the establishment of scriptural truth and were the very participants in critical salvation-historical events. There are no more 'Apostles' and 'Judges' as far as the Biblical definition go. Things are different now, we are not living in a time when God is giving new and special revelation not already familiar to what we have read in salvation history. We are all priests and prophets of the Lord so long as we are indwelt by the Spirit of God.
I personally do not believe in any 'call' of God to enter the ministry or mission field. The term call evokes the picture of God giving a subtle, whispering plea to special individuals and not to others. Those who are not 'called' can go about their merry way pursuing the vocation that they please while those poor Christian workers are subject to the onslaughts and trials that await them, beckoning them toward impending doom - their only consolation: you will receive treasures in heaven. There is no call in the sense that person A is supposed to engage in disciplemaking while person B need not.
The commission to make disciples of all nations is not an option for the follower of Christ. It is the very mark of what a disciple does - an absolute imperative. Taking up the cross and forsaking all for Christ's sake is not a suggestion, it's not advice!
Now, how one goes about fulfilling this command in their own individual lives precipitates into what we know as vocations and ministries. People go and do what they are gifted and led of the Lord to do. Some start businesses to advance the Kingdom of God, some go to jungle tribes, some open orphanages, others make movies. But where the explicit advance of the Kingdom of God is not seen in the vocation of an individual is where flesh and the world have cornered and choked the heart - syphoning off all evidence of personal redemption. It is to this type of man that Jesus says, "How difficult it is for the rich to enter the Kingdom of Heaven!" The only vocation that is not a ministry is the one belongs to a non-Christian.
What we see a lot of in Singapore (and not alot in the west) are businesses in marriage with religion. You will hardly walk into a family business without knowing the religion of that family. Sometimes worship music is playing in the waiting room of an x-ray clinic, or the sign board of the store reads, "John 3:16 Electronics". There's someone who doesn't need a calling to 'Christian ministry' in order to advance the kingdom of heaven. (Those in the market place making Christ known, I commend you. It's a difficult job, the casualties and compromise runs high).
Having said of all this, there is a downshot. Although not everyone is meant to be a cross cultural missionary/church planter to the unreached. God is looking for volunteers. You will be successful if you go as God's passion is the wind behind your back. If it's one thing the parable of the lost sheep, coin and son is meant to teach, it's that God's heart is hotly pursuing those who are lost. You would be hard-pressed to think of any need that is more critical and urgent to God's heart than the Kingdom that has yet to be established in so many places around the world. God's heart is restless within the city walls of Christendom as long as millions live in outer darkness on the other side.
People who say they are called to missions only say that so that they can feel special or so that people will get out of their way and not question their decisions. They can blame God when the going gets tough.
People who say "I am not called" are either afraid, lazy or completely unconcerned with what concerns God most.
The Kingdom of God and what you're doing to hasten its coming: Outside of this, there is no 'calling'