Maybe a firewall problem? Try a: tail -f /var/log/messages and/or tail -f /var/log/'your-firewall-log' and see if its rejecting or dropping any packets. If your convinced that your firewall settings are ok, reload the firewall once you are connected. Sometimes the firewall has to be reloaded when using ppp0, since the ip is dynamic. Maybe also try switching of the firewall completely, only to see if thats' why you cant do anything once connected.

Cheers.