>         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

Having designed a few of the solid state versions, I'll tell you.

The design requires loop current to bias on diodes and transistors that are
in the audio path. You shouldn't have to supply more than about 20mA, but I
base that on what the telcos guarantee to provide, I can't speak for the
designs of others.

Without sufficient current, you will get large attenuation of the signals,
and significant distortion. While a given modem may appear to work, it may
be on the hairy edge of failure due to line impairment.

One way to provide this current would be a DC power source, a resistor in
series (the current dosen't have to be regulated, but it does have to be
clean of hum) and a capacitor across the pair to keep the AC impedance low.
Ideally, the power source would be at least 24V, but with some equipment you
may achieve the minimum current with lower voltage. It varies, again
according to the specific design.