Romans 10:11-13 — For the scripture saith, Whosoever believeth on Him shall not be ashamed.
For there is no difference between the Jew and the Greek: for the same Lord over all is rich unto all that call upon Him.
For whosoever shall call upon the name of the Lord shall be saved.
Same quote as in Romans 9:33, from Isaiah 28:16 — … Thou shalt know that I am the LORD: for they shall not be ashamed that wait for Me.
Verse 12 — contrast with Romans 3:22 (all have sinned) now all can be saved
Verse 13 — quote from Joel 2:32 — And it shall come to pass, that whosoever shall call on the name of the LORD shall be delivered.
The same quote is used in Acts 2:19-21 where Peter refers to still-future Tribulation and blessing of all nations through Israel. He gives no indication of the age of grace because it hadn't yet been revealed through Paul. In Romans 10, Paul refers to the age of grace and salvation by Christ.
Putting all of these thoughts together, we may arrive at the conclusions that to call upon the name of the Lord is to believe all that the name of the Lord stands for; to know the Lord in His qualities as Savior God, Lord of all; to approach Him through the altar of the cross; to recognize that there is no strength in ourselves but that all power dwells in Him; and to commit ourselves to Him in faith, desiring that He should act for us as He sees our need. — Donald Grey Barnhouse, God's Covenant, page 85.