“If one were to try [using power from phone line], would phone company had a way of finding out?” Most assuredly. They aren’t in the business of supplying power, and they ARE in the business of finding faults in their lines. Any substantial power drain from their lines WILL be detected. If it’s large, the phone switch will conclude that you’ve dropped the phone in the bathtub or something like that, and will disconnect your line (and will check periodically to see if the drain has gone away and you can be reconnected). If it’s small, the switch will report it to the service people as a possible line problem, to be investigated before it causes a complete failure… and if they investigate and find that you’re to blame, they will probably send you a bill for time and trouble. The current you can draw without eventually having it noticed is very small.
- Henry Spencer (henry[at]zoo.toronto.edu)