First find out what kernel version you have by typing...

$> uname -r 

My kernel was...

2.2.0-idepci

Then use google to find the driver in a debian package for the ltmodem
which is what's in the compaq presario 1685...

Now install it using dpkg...

$> dpkg -i ltmodem-2.2.20-idepci_6.00c2_i386.deb

Then create a dialup connection using pppconfig, basically describing
your ISP...

$> pppconfig

Now make sure your ethernet and other junk is down by doing the following...

(if necessary)
$> ifconfig eth0 down

(if necessary)
$> ifconfig sl0 down

Then, in a new window you might want to see what the dialup connection is doing, so start tail, and it will display everything that happens at dialup time...

(in new window)
$> tail -f /var/log/messages

Now connect to your ISP (and watch the tail window)...

$> pon your_isp_name

When the tail window connects and starts showing send/recv messages, you're ready to surf the net...

My first recommendation is use the text-based internet browswer lynx, go to the opera internet browser website, download and install it's debian package, and finally you'll be ready to go!

$> lynx www.opera.com

To DISCONNECT from the internet, just type the following...

$> poff your_isp_name