I figured it out.
First you can set your external interface (eth0 or eth1, whatever) to be dhcp, just to get to the modem. It will give you a DHCP IP that is a local one (non-routable) 172.16/12 or 192.168/16.
You'll be able to access internet and all, but you still have a non-routable IP number.
Anyway, your network card will be configured as DHCP. If DHCP doesn't work for you, you can set up the route manually:
ifconfig eth0 up 192.168.1.77
route add -host 192.168.1.254
The latter is the address of the modem, the former is made up from the range of DHCP of the modem when it acts as a DHCP server. For your 172.16/12 network they would be similar -- figure them out. This setup worked for me.
Then you need to go to http://192.168.1.254/
and play with the ADVANCED settings of the modem. In the Configure->Connection set it to Bridged, and in Configure->DHCP turn it off -- you'll have reset the modem after each step -- it will ask you, just say yes.
This will make you modem be true DSL modem (just a modem).
Then you're ready to set up PPP. Assuming that you already have it installed on your machine this is how my /etc/sysconfig/network-scrips/ifcfg-ppp0 looks like:
USER=<your user id>@bellsouth.net
And my ifcfg-eth1 looks like this:
Then edit your /etc/ppp/pap-secrets and /etc/ppp/chap-secrets file
to have the proper email@example.com
and your password.
After all this you can restart the network:
After some wait, your ppp should start and you should be on the network.
Let me know if you have any questions.