What does it mean to `call upon the name of the Lord`? It means to come to the Creator calling Him by His name, Jesus, to plea for salvation.
God is just the generic name for deity.
Calling upon God will not save anyone.
Jesus is the express image of the only true and living God. It is only in the name, Jesus, that anyone is saved. -> Jesus is the name of His HolySpiritFather.
Understanding this gives real meaning to this important verse which declares the power of salvation in the name of Jesus.
When a person acknowledges that Jesus is Lord by calling on His name, His HolySpirit performs the work of salvation in the heart of the caller.