If your Cisco 7975G for example came without any power supply, and you aren't using PoE (Power Over Ethernet) then you may end up ordering a separate power supply with 48v DC output.

Here is an example, which proved to have a slight incompatibility. Although the electrical requirements match, with the phone labeled as requiring 48v @ 0.38A input, positive being at the center pin, and the adapter giving out 48v @ 0.375A,
the problem is the plug on the end of the adapter. At least on ones labeled as shown below. To fix it, you need to ground down a seemingly intentional solder point near part of the rubber boot, and the rubber boot as well for it to fit within the circumference of the receptacle plastic in the base of the Cisco phone. Perhaps there is a more common plug which would make a better fit instead.

ITE POWER SUPPLY
MODEL NO:LYD-4800375
OUTPUT:48V — 0.375A

MADE IN CHINA

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27-Aug-2009 replaced the above unit with a genuine Cisco CP-PWR-CUBE-3= model. These run far cooler, to the touch. The lousy ITE Power Supply mentioned up would run so hot that it most definitely didn't use a mosfet based conversion. I intend to break it open to determine what's really inside, and perhaps still hook it up to a power watt meter to see what the overall draw was.