Problem #1 - The Statement Assumes Modern Revelation
Within the question lies the implication that the individual who is now preaching has, at some point in the past, received some sort of revelation instructing him that his role in life is to preach the Gospel. While this may seem harmless to a cursory hearing, it is not a Biblical line of thinking.
In order for this to be true, modern revelation must still be a way by which God speaks to His people. However, this cannot be the case since the Bible clearly teaches that one does not receive instruction through such means after the completion of the God-given, inspired word of God, the Bible (1 Corinthians 13:8-10, Ephesians 4:8-16, 2 Timothy 3:16-17, 2 Peter 1:3). Even though the person may not believe modern revelation exists today, the question strictly enforces such a belief. A belief, I might add, which has lead much of the denominational world into the false doctrines and concepts plaguing the minds of those to whom the New Testament Christian evangelizes today. If you were to ask any evangelistic Christian today (though "evangelistic Christian" is redundant to say the least) whether or not the belief that God still speaks to and instructs individuals outside of the Bible is one of the main causes for religious error today, the answer would be a resounding yes!
Problem #2 - The Statement Misunderstands the Role of a Christian
The first problem may be a bit of nitpicking, but this problem is not. You see, the question of when a man received his calling misses the role of the individual Christian by a mile. It is commanded of every Christian to go and to preach the Gospel (Mark 16:15-16) and while there is a special role of some to be public proclaimers of the Gospel (2 Timothy 4:2ff, James 3:1) that position, if you will, is acquired based upon the heart and decision of the individual in question. It is not that the individual does not feel a strong urge to do so, for to say that is to say that the man is simply doing it because it's something to do which reminds me of the saying, "If you can do anything other than preach...do it!" But the drive to preach originates from the personal experiences (one reason why many preachers were once in denominational error), talents, love for the lost and intentions (bringing as many people to Heaven with him as possible) held by that man.
I am not meaning to suggest that God doesn't bring a man to preach by providential workings, because we cannot know the workings of God in His providing for His people and should not claim that we can, but that the decision to preach is not founded upon some "higher calling" but rather is founded upon the highest calling by which every person is called, the Gospel (cf. 2 Thessalonians 2:14). It is required that each Christian preach, some of us simply take it to the next level and make that requirement our vocation.
I hope that this article has been informative and encouraging to everyone who reads it. Even though I, or any other modern day preacher, did not receive a communication from a still small voice to go and preach I, and every other New Testament Christian, have the command to do just that. So next time someone asks, "When did you receive your calling" answer, "The day my Lord died on the cross for my sins!"