Dolphins can communicate, making this true. They have the power to communicate, and also, they are very smart. See this:
As an example
"Take Kelly. A dolphin who sadly lives in a research centre in the US, she has been trained to keep her tank clean. Every time she brings a piece of litter to her trainer, she is rewarded with a fish. So she’s built upon the idea. Now, when she finds a piece of paper, she wedges it under a stone, and tears off individual pieces, which she brings to the surface one at a time. Thus, a single piece of litter earns her several fish. She’s also noticed that gulls come to her tank, hungry for fish. So she uses one of her fish as bait, catches the unwary birds, and presents them to her trainers for even more food. She has not only created these remarkable strategies by herself, but she’s even passed them on to her calf."
There is proof that dolphins are smart.
Dolphins are the only other mammal outside of humans that have sex for pleasure. They have to be able to identify one another besides look, voices are the easiest way to do this. I think that if they only had sex for reproductive purposes I would disagree but because they do, I think that they are equipped and smart enough to detect voices from other dolphins apart.
Dolphins, Porpoises,and Whales have social familial order and hunt in packs. They have the ability to emit unique sounds both above and below the water. They can detect unique sounds emitted from others of their packs. Their intelligence may be higher than that of chimpanzees. They can swim long periods of time to track their missing members.
I think dolphins are smart enough to know each other's voices. Dolphins are highly intelligent animals and there's no reason to think this isn't possible. Dolphins have intelligence that is comparable to humans. I think we are able to do this, so it's not crazy to think a Dolphin could as well.
Dolphins are one of the smartest animals in the animal kingdom, and their communication abilities amongst each other are a known fact. They use a complicated system of clicks to communicate with each other and can distinguish between members of their group. This kind of communication is evident in their interactions with each other as well as with other animals.