Voice lines are recorded dialogue that can be activated during the course of a match. There are multiple variations per character and range greatly in their content and subjects, all of which directly correlate to the corresponding character. These too, are only unlockable through the random drops you receive from loot boxes. In my own experience, those who have unlocked them generally just repeat them while our team waits to begin an assault. Meanwhile, I am incessantly trying to score and sink a basketball in the net.
