People have been connecting weird stuff to telephone lines for about a
hundred and twenty years, and the companies are pretty much prepared
for this sort of thing.

Telephone systems have always been designed on the assumption that
wires will be 'cross-connected,' meaning that someone or something
will somehow connect telephone wires to power wires. The insulation
of the system is designed for a few hundred volts, which was necessary
for some old-fashioned frequency-division multiplexing they were
using.

The traditional protection devices are overvoltage protectors, which
used to be a carbon-electrode spark gap but are now (I think) gas
discharge tubes, plus overcurrent fuses known in telephone company
terminology as 'heat coils.' It's the latter that are the major
protection against power-line cross-connections, because while the
intermittent ninety volts 20 Hz AC that the central office generates
to ring your telephone is not vastly different from US power line
voltage, steady application of power-line voltage will force
considerable current through a telephone receiver for an extended
period. The heat coil is calibrated to open after a short time under
such conditions.



M Kinsler