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If you unplug the wire connected to your Plain Old Telephone and plug the
four pin modular plug into an eight pin modular jack used in Ethernet (yes
it fits and has proper electrical and mechanical functionality) the DC
voltage is measured between pins 4 and pin 5 with an open circuit voltage
of about 48v.

The Nortel Meridian phones use the same 4/5 pairs, in the same way, but the
voltage is around 30 volts.
The Meridian 4/5 pair have DC voltage which is symetrical about ground as
opposed to the plain old telephone which is ground and -48v .

Just some information for the pile

Tom Gandy
Industrial Catalyst