Some great insights into God's call from Oswald Chambers' "My Utmost For His Highest" devotional:
"Called By God": "I heard the voice of the Lord saying: "Whom shall I send, and who will go for Us ?" Then I said, Here am I ! Send me." - Isaiah 6:8
God did not direct His call to Isaiah— Isaiah overheard God saying, ". . . who will go for Us?" The call of God is not just for a select few but for everyone. Whether I hear God’s call or not depends on the condition of my ears, and exactly what I hear depends upon my spiritual attitude. "Many are called, but few are chosen" (Matthew 22:14). That is, few prove that they are the chosen ones. The chosen ones are those who have come into a relationship with God through Jesus Christ and have had their spiritual condition changed and their ears opened. Then they hear "the voice of the Lord" continually asking, ". . . who will go for Us?" However, God doesn’t single out someone and say, "Now, you go." He did not force His will on Isaiah. Isaiah was in the presence of God, and he overheard the call. His response, performed in complete freedom, could only be to say, "Here am I! Send me."
Remove the thought from your mind of expecting God to come to force you or to plead with you. When our Lord called His disciples, He did it without irresistible pressure from the outside. The quiet, yet passionate, insistence of His "Follow Me" was spoken to men whose every sense was receptive (Matthew 4:19). If we will allow the Holy Spirit to bring us face to face with God, we too will hear what Isaiah heard-"the voice of the Lord." In perfect freedom we too will say, "Here am I! Send me."