Once a PPP link is operational (specifically, once the IP layer is operational), Linux PPP can automatically run (as the root user), a script to perform any function you can write a script to accomplish.
Section Chapter 25 provides information on the /etc/ppp/ip-up script, the parameters it receives from PPP, and how to use it to do things like, acquire your email from your ISP account, send any queued email waiting transmission on your machine and such.