1) About 50 volts floating around 10 volts when off hook (decreases as more stations pickup in a bridge) and right around 90 volts during ring.
2) There is very little difference in the voltages on the PSTN than the PBX, usually things will be a bit lower at your house because you're further away from the CO than you are from the PBX at work, also the carrier might use some additional equipment in the line depending on how far its going.
3) See the end of 1... enough to make ya feel it.
