[numpad0]

Looks like PoE uses pin 4 and 5 for +48V, and 7 and 8 for GND, which respectively are pair C and D of GbE. On ISDN, pinout could vary but one I could find said 1-2, 4-5 for V+ and 3-6, 7-8 for V-. In either cases, both sides of isolation magnetics are connected to pins of same potential and current is only proportionate to voltage imbalance within each pair which should be minuscule.

I wonder if the problem is that 1:1 signal transformers for decoupling are being replaced with simple DC blocking circuit. That can be most simply done with a spare 0.1uF and an R10k per pin, which by the way generate load of from V/R=I 48V/10kohm = 5mA and 48V * 5mA = 230mW > 1/6W. That could cause resistor to burn off if phone-sized components would be used. Or if the cap might only be rated for 10V, it could burn open. I have nothing to support these hypothesis though. I could be entirely off.

Also I suspect the "passive PoE" mentioned above could be cheap injectors with very rounded corners and always active 48V. Those are widespread for surveillance cameras and other neckbeardy applications.

[toast0]

> Looks like PoE uses pin 4 and 5 for +48V, and 7 and 8 for GND, which respectively are pair C and D of GbE.

There's three common pin outs. You've described 'alternate B' -- the unused pairs of 10/100; you can also use 'alternate A', using the pairs used for 10/100. Or 4-pair would have both.