Many people have problems getting PPP to work straight away. The variation in PPP servers and how they require you to set up the connection is enormous. Similarly, there are many options to PPP - and some combinations of these just do not work together, ever.
In addition to the problems of logging in and starting the PPP service, there are problems with the modems and the actual telephone lines as well!
Section Chapter 8 provides some basic information about common errors, how to isolate these and fix them.
This is NOT intended to provide more than just the basics. Al Longyear maintains the PPP-FAQ which contains much more information on this topic!