USE USB!!!!... Its a 5 volt power source for a 5 volt charged device!
Dont bother with power off of ethernet NIC. Also if you use an adapter that has a PoE source you will smoke that phones charging circuit with over voltage and possibly have a lithium fire.
https://en.wikipedia.org/wiki/Power_over_EthernetIf you have a shortage of USB ports, add a USB adapter card inexpensively. Or add a USB HUB.
On a mod I did about 10 years ago I mounted a router and switch to inside of a desktop computer and powered them off the 12V extra P-connector off of the unused P-connectors from computers internal power supply for a lan party system where others connected to my box, but I wouldnt advise using a direct 5V tap to an unused plug as for its 5 Volts with a boat load of amps behind it and if something goes wrong with powered off of USB, the USB will overload and stop power to the device, but if something goes wrong with direct 5 volt internal power you can have a fire!