Is this one of those units that sends the telephone over house wiring?

Bill wrote:
I consider myself technical, but this one baffled me....

Working on some oak trim molding, I unplugged a phone jack of a nearby phone
(which also has an AC outlet, that remained plugged in).

- Several hours later, I try to make a call and get NO dial tone from any
phone in the house.
- I start to unplug one phone at a time, seeing where the culpit is. Still
NO dial tone.
- All phones disconnected, I don't even get a signal at the main feed into
the house.
- Convinced it was "OUTSIDE" I called SBC service to "check my line".

After some discussions, I find the problem was I never disconnected the AC
outlet for the one phone that I originally unplugged the phone jack.

Once the AC outlet for that phone was unplugged. I had full dial tone. The
service rep says voltage was TRAPPED in the phone line, even though the
phone jack was disconnected. Somehow unplugging the AC plug "frees" up the
trapped voltage in the phone lines.

The 2 circuits seem completely independant. How is this possible???