It may let the rest of the pod know who is around, where they are, and perhaps, something about their mental state. Dolphins in distress sometimes emit their signature whistles very loudly. Whistles may serve to establish or maintain vocal or physical contact between dolphins. Dolphins often respond to another dolphin whistling by whistling themselves or moving toward the whistler. If they become separated a young calf and its mother whistle frequently until reunited.
Dolphins also whistle when separated from other group members.
Dolphins do not always respond immediately to another dolphin whistling. Sometimes, many dolphins in the group whistle at once, repeating their signature whistles over and over. In this case, the whistling may help the dolphins keep track of where others are. Dolphins are capable of imitating certain sounds very accurately and often learn other dolphins' whistles. One reason for imitating another dolphin's whistle may be to make contact with that dolphin and get its attention within a large group.