no voltage on POE port

In due course I got this from Netgear tech support:

"During the detection phase the PSE relatively slowly (0.1 V/ms) ramps up the power from 0 to 10 V. During this time, the current allowed to flow is limited to 5 mA in case there’s a short or other problem. As the voltage rises, the PSE measures the current flow at two levels (at least 1-V apart) to determine the possible presence of the detection signature resistor in a PD at the other end. Two measurements are required to factor out the DC offset (cable loss) between the PSE and PD. If the PSE detects an open circuit (i.e. nothing or a non-PoE device plugged in), short circuit, or anything other than the specified detection signature resistance, it turns off the power. 

Assuming that a valid PD is found (a valid detection resistance), the PSE continues ramping the voltage. As it passes between 15.5 and 20.5 V, the PSE and PD enter what’s known as the classification phase, in which the PD is given the opportunity to request a particular amount of power. Once again, the technique involves the PSE measuring the current flow, this time across a classification resistance. Note that from a PD’s perspective, the classification phase is optional with full power the default. However, a PD that volunteers to accept a lower power level will be welcomed by multiport PSE’s that must allocate their maximum power delivery capability across many ports. Knowing now that there is a PD requesting power (and how much it requires), the PSE ramps the voltage and current limit to their full specifications. 

The PD must be able to accept power on either the data or spare pairs (but never both at the same time). The IEEE standard specifically disallows PDs that don’t accept either power-delivery option. "

This was helpful in clarifying how the PSE is supposed to work.  What is still not clear is why a 25K resistor across the spare pairs does not cause the GS308P to source the default 48 volts.

I conclude that it would be helpful is the product documentation stated exactly what the product does and how to test it.  (Given that PDs have a range of behaviours just plugging one in would not prove much).