Create CHAP credentials for additional trusted callers 

How to Create a CHAP Credentials Database (Dial-in Server)

Update the PPP configuration files to support CHAP 

How to Add CHAP Support to the PPP Configuration Files (Dial-in Server)

Set up CHAP authentication on the dial-out machines of trusted callers 

Configuring CHAP Authentication for Trusted Callers (Dial-out Machines)