etp72 - When you plug a device into a standard wired RJ-11 or RJ-45 jack it connects via the center pins.
On a prewired cat5 cable the blue & blue white wires are wired to the center pins. But, it does not matter because, both ends are the same. The center pins on one end connect to the center pins on the other end.
If one end is wired (no standard plug on that end) make sure you have found the circuit you want to connect on the existing cable & connect that to the pair that is going to end up on the center pins of a standard plug or jack, as in your wall plate.
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