Landline phone systems rely on PSTN (aka traditional circuit-switched phone networks) to make calls happen. That’s not the case with VoIP, which uses packet-switched networks.
PSTN (circuit switching) - When person A calls person B, the network tries to find a connection from different circuits (sort of like paths or trails with gates). Once a connection is established, the conversation takes place and the circuit stays open until the call is finished.
VoIP (packet switching) - When person A calls person B, person A’s voice signals are broken down into data packets (as explained above). Each packet contains the source and destination addresses (where the message comes from and where it’s going). This is important, since the packets may take different routes. When they arrive, the destination device (phone or computer) reassembles them into person A’s original voice signals. The same thing happens to person B’s voice signals as the conversation goes on.