I've been trying for a while to get my SpeedTouch ADSL modem working. I'm following the steps of this howto:
http://linux-usb.sourceforge.net/SpeedTouch/howto.html
There is a troubleshooting appendix on that site, and I have been able to figure out from that why my modem's not working. But I don't know how to fix it. Look at this:

Check your console or /var/log/messages file to see when ppp says.
If it says something like this:
pppd[1234]: LCP: timeout sending Config-Requests

Go to /proc/net/atm and do:

cat "/proc/net/atm/Alcatel SpeedTouch USB:0"
If this file is not there, the speedtouch module is missing. Repeat step 5.
The output should be something like:
Speed Touch USB:0 (00:90:0d:xx:xx:xx)
AAL0: tx 0 ( 0 err ), rx 0 ( 0 err, 0 drop )
AAL5: tx 0 ( 0 err ), rx 0 ( 0 err, 0 drop )
If the numbers are all 0's, the ppp daemon is wrongly configured and it is not sending data to the modem.
If the numbers are NOT zero, check whether you have used the correct VP/VC settings and the correct protocol!

That's exactly what's happening, and cat'ing that file gave me all 0's, like above. So pppd is wrongly configured, how do I configure it?