> can 2 modems communicate w/o supplying the canonic 30 volts to the
> connecting copper pair ?

        Some can and others can't, I believe it has to do with the type of modem
 to
line interface used. Every modem I have with a real transformer works fine
without any line voltage or current, every modem that uses some solid state
device on the other hand does not.

> If not, why not ?

        Not sure on that one. TTYL