In the New testament, the phrase "the sons of Israel" is used 15 times.
In several of those places it is referring to the Jews of the old testament.
( Matt 27:9; Luke 1:16; Acts 7:23; Acts 7:37; 2 Cor 3:7; 2 Cor 3:13; Heb 11:22; etc... )
A couple of verses are talking about the 144,000 who are also from the sons of Israel ( Rev 7:4; Rev 21:12; )
In Acts 9:15; Luke says that Paul is chosen to bear Jesus' name before the Gentiles and the sons of Israel.
So obviously the sons of Israel aren't the Gentiles.
In fact.. it seems not all of the sons of Israel will be saved.. only a remnant ( Rom 9:27; )
So if the sons of Israel are all believers... then why would only a remnant of them be saved?
I really see no place in the new testament where Gentiles are called the sons of Israel.
The one passage that seems to cause some confusion here is in Rom 2:28-29;
But even here, it doesn't use the phrase "sons of Israel, it simply says "a Jew".
Jesus also seems to separate the house of Israel from the Gentiles. ( Matt 10:6; Matt 15:24; )