That's because desktops usually come with full-size parts that are often more powerful than the similar counterparts designed for laptops. It's especially noticeable with processors and a computer's overall speed.
An IP phone connects through an Ethernet cable, rather than a phone jack, to the internet, where it connects to the IP address of either the VoIP provider or the hosted PBX provider.
A switch is a device in a computer network that connects other devices together. Multiple data cables are plugged into a switch to enable communication between different networked devices.