A network restart or reboot is one of the simplest yet most effective troubleshooting tools. Use this technique when all phones on the network are affected, particularly when the issue started suddenly. Some networks rarely (if ever) need to be restarted, while some must be restarted once a week or more. It is almost always necessary to perform a proper restart after a power outage or an ISP outage.
It is recommended to have someone familiar with the local network perform the network restart.
Warning: Do not use a button labeled "reset" or "restart" since this button may restore the device to the factory default settings. A button labeled "power" is fine to use, but unplugging the power cables would be the best option.
Modem --> Router --> Switch --> Phones
Modem --> Router --> Phones
Modem/Router --> Switch --> Phones
For more information on how to optimize performance, see the following articles: