Concise Bible Dictionary:

The words καλέω, κλῆσις, κλητύς have various applications in scripture. There is
1. the usual position or occupation of a person, as slave or freeman: the Christian is exhorted to continue in his calling if he can do so with God (1 Cor. 7:20-24).
2. The general “call” or invitation by the gospel, in contradistinction from those that are “chosen” (Matt. 20:16; Matt. 22:14).
3. God’s call to individuals, when he also makes them willing to obey: as when Abraham was called to leave his country and kindred (Heb. 11:8).
4. In an absolute sense for salvation: “whom he did predestinate, them he also called: whom he called, them he also justified” (Rom. 8:30; Rom. 11:29). The saints are saints by calling; the apostles were apostles by calling (Rom. 1:1,7). The Christian is exhorted to use diligence to make his “calling and election” sure (2 Pet. 1:10), evidently not in the mind of God, but in his own mind.
5. We read of the “high” calling, the “holy” calling, and the “heavenly” calling (Phil. 3:14; 2 Tim. 1:9; Heb. 3:1). The “vocation” in Ephesians 4:1 is the same word.