I have 4 pair (8 wire) telephone cable running all through the house. One pair of wire is for the telephone. Is is possible to use the three remaining pairs (6 wires) as an ethernet cable?

A phone cable is usually made from 0,6 mm2 copper wire. He is using a 160 mA fuse before the transformer and 125 mA fuse before the plugs. So really little current can get into the phone cable.