Does human merit have any part in God's saving and calling?

No. Human merit has no part in God's saving and calling. Scripture says God saved us and called us with a holy calling, not according to our works, but according to His own purpose. We were not called because we earned something.

Les explains that Paul, writing to Timothy—a Gentile believer, though just as well writing to us—says God has saved us and called us with a holy calling, not according to our works. Les emphasizes that our merit doesn't mean a thing; we were not called as people who earned something. The call goes out to every son of Adam, but it is not according to our works; it is according to God's own purpose. If you are a child of God, it is because it was in the purpose of God. You were saved for a purpose and a reason, already in the mind of God before He ever created Adam.

2 Timothy 1:9

Who hath saved us, and called us with an holy calling, not according to our works, but according to his own purpose and grace, which was given us in Christ Jesus before the world began,

Why did God save and call us — is it because of our works or His own purpose and grace?

God saved and called us with a holy calling, not according to our works, but according to His own purpose and grace. That grace was given us in Christ Jesus before the world began and is now made manifest by the appearing of our Savior, who abolished death and brought life and immortality to light through the gospel.