To find what is going wrong it may be useful to log messages from pppd, You can find information about how to do it at the files FAQ and README in the directory /usr/doc/ppp-2.4.1/
A quick resume is:
Add this line to /etc/syslog.conf:
note: only use <TAB>, don't use the spacebar.
and create an empty file /etc/ppp/log
To reread the changes made find the PID of syslogd typing: ps -x
and type: kill -HUP "PID of syslogd"
Start your connection with the command ppp-go and it will log packets send and received. Examine this log to see whats happening.