I used the Cermetek CH1770 in a design about ten years ago.  It
was 1200 bps and had its own strange command codes.  If you've got room,
and the I/O pins, how about putting an 8 bit ISA socket on your board and
buy whatever internal modem board is available at the time.  These are
VERY inexpensive, and are FCC approved (you'll spend more money on an
approved DAA if you go with the modem chip approach).  As better modems
come out, you just plug them in!

Harold