How was Paul called by God?

Paul was called by God's grace, set apart from his mother's womb, when it pleased God to reveal His Son in him. He did not confer with flesh and blood or go to Jerusalem, but went into Arabia, that he might preach Christ among the Gentiles.

Les teaches that Paul was a religious Jew who was doing everything the Book instructed, but when it pleased God, who separated him from his mother's womb and called him by His grace, the purpose was to reveal His Son, the Lord Jesus, in him, that he might preach Him among the heathen, the Gentiles, the non-Jew world. Les explains that Paul did not go to Jerusalem to check in with Peter; he conferred not with flesh and blood, neither did he go up to Jerusalem to them who were apostles before him. Instead, he went into Arabia. Les points to Galatians 4:25, where Hagar is Mount Sinai in Arabia, to show what was in Arabia.

Galatians 1:15

But when it pleased God, who separated me from my mother’s womb, and called me by his grace,

Did Paul deserve to be called by God?

No, Paul did not deserve to be called by God. He was a zealous religious leader who profited in Judaism, yet God separated him from his mother's womb and called him by His grace. If anyone did not deserve it, Saul of Tarsus did not, but God called him by grace to preach to the Gentiles.