A PoE switch is a network device that can provide both data and power to other devices over an Ethernet cable. PoE stands for Power over Ethernet, which is a technology that allows electrical power to be transmitted along with data on twisted pair Ethernet cabling. This eliminates the need for separate power supplies or AC adapters at the users' desks, as the VoIP devices can receive power from the PoE switch.
A firewall is a network device that monitors and controls incoming and outgoing network traffic based on predefined security rules. It does not provide power to other devices. A power injector is a device that adds electrical power to an Ethernet cable for a single device. It is not suitable for a large number of devices, as it would require one power injector per device. A router is a network device that connects multiple networks and forwards data packets between them. It does not provide power to other devices.