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The Cisco UC 520 has 8 PoE ports out of which you can draw 88 watts of power. This means that with each Cisco IP phone drawing 12 watts of power, you can only have 7 IP phones (= 84 watts) on the 520.
If you don’t know this it can be quite hard to troubleshoot, because the phone doesn’t get enough power to create a link, so it looks as if the port itself is dead.
If you are planning to implement the UC 520 in an environment with exactly 8 phones, remember to add the companion switch to the solution so that you have enough power to support all phones.