This document provides guidance on how to configure your Linux PC as a PPP server (allowing other people to dial into your Linux PC and establish a PPP connection).
You should note that there are a myriad of ways of setting up Linux as a PPP server. This document gives one method - that used by the author to set up several small PPP servers (each of 16 modems).
This method is known to work well. However, it is not necessarily the best method.