Before attempting to use PPP, you must check two things.
There are no instructions here for troubleshooting serial links.
Test the modem installation using a terminal program such as
minicom. If you can dial the modem, and log into a remote site,
(DAS, for example) the modem should work fine.
At a minimum, you need: ifconfig, and route.
These will suffice to get the link set up.
To actually use the link, you'll need things like telnet,
rlogin, a web browser, an ftp client, or the like.