Also, do check the pins on your Dreamcast modem, mine had stopped working recently, and after confirming my LVI works with another modem, I disassembled it and noticed the pins had shifted in position. Instead of having 4 pins going straight, I had 3 pins shifted so that the 3rd pin was sitting on top of the 4th.
They're pretty easy to correct after disassembling the modem though:
(Unlike in picture, RJ11 cable must be disconnected when moving the pins!)
Although, my modem still doesn't work after correcting the pins :/