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Connect the UPS to an available outlet.
Consult the UPS device's user manual in order to reference the meaning of any status lights on the device. Most devices will have a green LED light that turns on when the UPS is operating correctly. Before you connect any equipment to your UPS, use these status lights to verify that the UPS is fully operational and prepared to power other equipment.
From a keyboard and monitor attached to your server, login as root
Type shutdown -h now from the root console
The system should display a number of shutting down messages to the console. At the very end of this output, the system will say Power Off
At this point, please turn off the power to the system. Either press the rocker switch on the back of the box, or press the power button on the front (it depends on the type of system you purchased)
Connect the power cable from your server to an available outlet on the UPS
If there are status lights indicating a successful connection, verify these lights turn the appropriate color indicating no issues with the UPS power port (the light will typically be green on most devices)
Once you have verified that the physical connections are good, power on your server again.
Your server should boot normally. Try to make a test call from one of the phones in your office to an outside number to determine if everything is operational once again (allow the server enough time to boot before you try to make a call - making a call immediately after you press the power button will naturally fail as the server will not have completed it's boot sequence)