Objection 1. It would seem that all the angels know what one speaks to another. For unequal local distance is the reason why all men do not know what one man says to another. But in the angelic speech local distance has no effect, as above explained (4). Therefore all the angels know what one speaks to another.
Objection 2. Further, all the angels have the intellectual power in common. So if the mental concept of one ordered to another is known by one, it is for the same reason known by all.
Objection 3. Further, enlightenment is a kind of speech. But the enlightenment of one angel by another extends to all the angels, because, as Dionysius says (Coel. Hier. xv): "Each one of the heavenly beings communicates what he learns to the others." Therefore the speech of one angel to another extends to all.
On the contrary, One man can speak to another alone; much more can this be the case among the angels.
I answer that, As above explained (1,2), the mental concept of one angel can be perceived by another when the angel who possesses the concept refers it by his will to another. Now a thing can be ordered through some cause to one thing and not to another; consequently the concept of one (angel) may be known by one and not by another; and therefore an angel can perceive the speech of one angel to another; whereas others do not, not through the obstacle of local distance, but on account of the will so ordering, as explained above.
From this appear the replies to the first and second objections.
Reply to Objection 3. Enlightenment is of those truths that emanate from the first rule of truth, which is the principle common to all the angels; and in that way all enlightenments are common to all. But speech may be of something ordered to the principle of the created will, which is proper to each angel; and in this way it is not necessary that these speeches should be common to all.
Source: Summa Theologica (New Advent)