I had some problems before with ppp
Try some of this:
to check if you actually have the gateway configured by the DHCP server.
Code:
more /etc/reslov.conf
This will check if you have a DNS server
as kencaz recomended to see if the DNS on the /etc/resolv.conf is active.
If you follow all these and they seem to be ok, call the ISP because is not on your end man!!!.
You can also try the iptraf to be positive if there is any traffic at all.