Addressing God as "Lord" does not, in itself, mean that the person is calling upon the name of the Lord in a saving way.
In Matthew 7:21, the surrounding verses are clear that Jesus is warning some who think they are doing good things in Jesus' name, that he will not recognise them as his, on that day of Judgment he speaks of. He will cast them aside where they will remain outside of the kingdom of heaven. When those ones stand before Christ for judgment, they will start out by addressing Christ as "Lord, Lord," only to be told the shocking truth - that they never confessed him the way Jesus accepts.
A similar point is in the O.T. in Isaiah 59:2:
"Surely the arm of the Lord is not too short to save, nor his ear too
dull to hear. But your iniquities have separated you from your God;
your sins have hidden his face from you, so that he will not hear."
(NIV, 1987 edition)
Those people were calling upon the Lord God, confident that they were his people, yet God chose not to hear them, because of their sin. Their calling would not save them. Sin that is not repented of renders any response to the call to worship futile.
This shows that "calling upon the name of the Lord" that the N.T. speaks of, which brings salvation, has a particular meaning. You have quoted two such texts, Acts 2:21 and Romans 10:13. They speak of the call of faith, the call of worship, with resulting obedience to the call of God and of Christ.
Will the Lord save anyone who will not worship him? No, he will not. But worship that is not based on faith is ritual, and is not the kind of worship Jesus said must be given.
"The hour comes, and now is, when the true worshippers shall worship
the Father in spirit and in truth, for the Father seeketh such to
worship him. God is a Spirit, and they that worship him must worship
in spirit and in truth." John 4:23-24
Jesus pointed out that it was not location that had a bearing on God hearing the calls of such worshippers. It is calling on the Lord "in spirit and in truth" that saves the caller. Worshippers can say many things (Matthew 7:21) but calling upon the Lord's name in a saving way requires faith, and it is only saving faith that... saves.
Your concluding verse in Acts 2:39 nicely shows what this kind of "calling" means: God calls to those he saves; they hear his call and respond in faith by calling upon the name of Jesus to be saved. All such will be saved. "Deep calls to deep..." (Psalm 42:7). This matter of "calling" is a deep matter, indeed.