During their first 6 months, infants will often coo or babble while their caregivers are talking to them. By 7 to 8 months, babies are more likely to show vocal turn-taking: remaining quiet while someone speaks to them and making their own vocal responses when the person finishes. Babies may learn this type of turn-taking from adults, who usually speak to a baby, wait for a coo or gurgle in response, and then speak again to the baby.
Rosenthal, M. K. (1982). Vocal dialogues in the neonatal period. Developmental Psychology, 18, 17
Snow, C. E., & Ferguson, C. A. (Eds.). (1977). Talking to children. Cambridge, UK: Cambridge University Press.