By default, pppd will log any warnings and error messages to syslog's daemon facility. You have to add an entry to syslog.conf that redirects this to a file, or even the console, otherwise syslog simply discards these messages. The following entry sends all messages to /var/log/ppp-log:
If your PPP setup doesn't work at once, looking into this log file should give you a first hint of what goes wrong. If this doesn't help, you can also turn on extra debugging output using the debug option. This makes pppd log the contents of all control packets sent or received to syslog. All messages will go to the daemon facility.
Finally, the most drastic feature is to enable kernel-level debugging by invoking pppd with the kdebug option. It is followed by a numeric argument that is the bitwise OR of the following values: 1 for general debug messages, 2 for printing the contents of all incoming HDLC frames, and 4 to make the driver print all outgoing HDLC frames. To capture kernel debugging messages, you must either run a syslogd daemon that reads the /proc/kmsg file, or the klogd daemon. Either of them directs kernel debugging to syslog's kernel facility.