There is a two-fold call.
(1) There is an outward call, which is nothing else but God’s blessed tender of grace in the gospel, His parleying with sinners, when He invites them to come in and accept of mercy. Of this our Saviour speaks: ‘Many are called, but few chosen’ (Matt 20:16). This external call is insufficient to salvation, yet sufficient to leave men without excuse.
(2) There is an inward call, when God wonderfully overpowers the heart, and draws the will to embrace Christ. This is, as Augustine speaks, an effectual call. God, by the outward call, blows a trumpet in the ear; by the inward call, He opens the heart, as He did the heart of Lydia (Acts 16:14). The outward call may bring men to a profession of Christ, the inward call brings them to a possession of Christ. The outward call curbs a sinner, the inward call changes him.