(1 Corinthians 1:1) Παῦλος κλητὸς ἀπόστολος Χριστοῦ Ἰησοῦ διὰ θελήματος θεοῦ καὶ Σωσθένης ὁ ἀδελφὸς
(KJV) Paul, called to be an apostle of Jesus Christ through the will of God, and Sosthenes our brother,
Note that “to be” is added in the English and does not appear in the original Greek. Thus, on the one hand, it would seem that “a called apostle” is a more literal translation of κλητὸς ἀπόστολος.
On the other hand, perhaps there is a rule of Greek grammar which allows κλητὸς ἀπόστολος to be interpreted as “called to be an apostle.” (Kind of like how a sentence in Hebrew is complete without needing a copula.)
To me, the two possible translations don’t have the same meaning, although the difference is subtle. Perhaps I am “called to be a professor,” but unless I actually become one as well, I’m not a “called-professor.”