It's been a few years since I set one up, but I always got burned
on the tty device permissions (which is not your problem) and the
way the modem deals with Carrier Detect.  As I remember, I had to
have CD set to stay up and just briefly drop when a call ended.
(AT&D3 or something like that.)  Of course the fun part of that
was that the computer wouldn't talk to the modem until it was
set that way...  You might need to get a breakout box and tie off
the CD line to be able to change the setting with the sparc  (or
the PC which probably ignores the line can do it.)

Jon