Click here to go to a mirror of this page. It contains hardware specific information for Conexant modem chips.
The Dreamcast's modem uses the chips marked RP56LD for the 56kbps version and RP336LD for the 33.6kbps version.
Also, here's the document for the MCU which is used for executing code loaded into SRAM. It's based on the Rockwell 6502 processor but with some modifications.