Plain Old Telephone Service (POTS) has approximately 50VDC on the pair. There are some services that can put up to 130VDC - 190VDC on the pair (total reach ISDN), but that's about as high as we go, and if you or your neighbor doesn't have such an animal, then you're back to 50VDC. There can be induced AC on the line, too, although we do take steps to mitigate this as much as possible.