There are two models of the modem adapter, 670-14140A and 670-14140B. The "A" model can use power from the Dreamcast game console to allow it to operate without the need of power from the telephone line. The "B" model does not use power from the Dreamcast thus being dependent on the power from the telephone line or a modem that runs power through the telephone line.
...the modems with "A" on the end of the model number use power from the DC, so, do models with the A at the end require an LVI?