Circuit switching (the basis of traditional phone service) has been around for more than a 100 years and might have been around a hundred more if packet switching hadn't come along.
VoIP (Voice over Internet Protocol) uses packet-switching technology. Packet switching, a way of breaking up a phone conversation and transmitting it in a way that eliminates wasted space, is a vast improvement over the old method.
A packet-switched network sends and retrieves data only as you need it. While circuit switching sends your phone conversation over a dedicated line (no one else is using it), a packet-switched phone conversation flows through a frenzied network along thousands of possible paths.
Circuit switching keeps the connection between you and the person you're calling open and constant. Packet switching opens a brief connection that's just long enough to send a small piece of your conversation (a chunk of data called a packet) from one system to another. And if you're not talking, it's not sending.
When the multitude of packets (traveling a multitude of paths) gets to its destination, the packets of conversation are reassembled into the original form. And you didn't even know it happened.