"But when it pleased God . . . to reveal His Son in me . . ."
--- Galatians 1:15-16
The call of God is not a call to any particular service; my interpretation of it may be because contact with the nature of God has made me realize what I would like to do for Him. The call of God is essentially expressive of His nature; service is the outcome of what is fitted to my nature. The vocation of the natural life is stated by the apostle Paul - "When it pleased God to reveal His Son in me that I might preach Him" (i.e., sacramentally express ) "among the Gentiles."
Service is the overflow of super abounding devotion; but, profoundly speaking, there is no call to that, it is my own little actual bit and is the echo of my identification with the nature of God. Service is the natural part of my life. God gets me into a relationship with Himself whereby I understand His call, then I do things out of sheer love for Him on my own account. To serve God is the deliberate love-gift of a nature that has heard the call of God. Service is expressive of that which is fitted to my nature: God's call is expressive of His nature; consequently when I receive His nature and hear His call, the voice of the Divine nature sounds in both and the two work together. The Son of God reveals Himself in me, and I serve Him in the ordinary ways of life out of devotion to Him.