one tip(!)...

not sure about the 1+ but on the classic, the parallel and serial ports are 
*both* female D25 type. connecting serial boxes to the parallel port can blow 
them due to the higher voltages (i've done this with a brand new 3com 
superstack haha what an idiot!).

first thing to try is tip to your modem. set up a modem line in /etc/remote 
(see man pages for tip and remote) and see if you can tip to it. make sure 
your modem is happy with the parity. usually no parity but some prefer even. 
if all goes well you should be able to get as far as dialing from this 
connection. ppp wont work this way tho. note the mail from paul regarding the 
speed limitations of old sparc serial ports.

once you can talk to yer modem ppp should be straightforward to set up.

GREG