Try using the wvdial tool. There are also two graphical frontends that use wvdial called gnome-ppp and kppp.
To set up wvdial run these commands in the terminal:
This writes the initial file.
Then edit the file to enter the phone# you dial out on, username, and password:
make sure there are no semicolons, or extra charcters at the beginning of the phone, username, password lines. Save the file. Now run:
gksu gedit /etc/wvdial.conf
if all goes well, you will connect and be able to open your browser.