People have different philosophies about ministry calling. "Calling" is a word used to denote a special direction from God towards doing Christian ministry work. Some people believe this "calling" has to be as dramatic as Moses and the burning bush or you'll never survive in ministry's tough times. Others believe anyone and every Christian is a minister in a sense, no matter what they're job title happens to be at the time.
I grew up in the camp of "calling" being a special leading from God at some dynamic moment of your life. I've learned and taught that God will speak to you and put a ministry calling in you heart, and will then confirm what you feel led to do through prophetic words from others throughout your life as well. God always confirms his words to us.
It was seemingly easier in the Apostle's day. Jesus simply spoke to them saying "follow me," and "your an apostle." At that time he was flesh and bone and had an audible voice, whereas now we depend on our openness and ability to hear and interpret the Holy Spirit of God.
There is another principle in Scripture, however, that I believe is often overlooked is 1 Timothy 3 - "It is a true saying that if someone wants to be an elder, they desire an honorable responsibility. For an elder must be a person whose life cannot be spoken against." Elder to the early church meant a pastor or minister. In essence, Paul is saying two things are required for calling - 1. a sincere desire to do it by a Christian, and 2. he continues on to lay what moral standards must make up that person's life. They must be exemplifying the things they want to teach as a minister.
In the early 1900's a twenty something by the name of Evan Roberts had this vivacious desire to be in ministry, but he had never heard the call of God in his heart. He was desirous to grow other's spiritually, but had never encountered a "burning bush" or other phenomenon. His Bible was well read year after year and after half a decade of this ministry longing, he decided to at least attend Bible college. Within one semester of schooling he dropped out because in prayer God was doing so much and giving him such a great vision. He began to preach at churches (without an official "calling," but with a great desire and a holy life) and soon God was using him all over Wales. History calls the movement that followed the Welsh Revival, a prelude and spark to the Azusa Street revival and eventually the charismatic renewal here in the US. I am especially grateful to his giving into this desire to do ministry because God using him had a major part with launching the denomination that I grew up in and am a part of - the Assemblies of God.
I still believe God calls people in special ways, but there is something about that desire in a person's heart for ministry with a holy lifestyle to back it up that can't be forgotten as well.