Packet-switched networks operate differently. Data streams are broken into small packets that march like a stream of ants across the network, often taking different routes to their destination, where they are then reassembled. All traffic streams share the same wires at the same time, which is good inasmuch as it substantially increases the carrying capacity of the network.
This works fine for data transmissions, but less well for the quality of IP voice calls, which require a smooth, uninterrupted, low-latency stream. You have no control over what happens after your VoIP packets leave your network, but at least you can give them a good start. You have a lot of control over your LAN performance, which is important for your overall VoIP quality.