You should first read the ppp(8) man page and the PPP section of the handbook. Enable logging with the command
set log Phase Chat Connect Carrier lcp ipcp ccp command
This command may be typed at the ppp(8) command prompt or it may be entered in the /etc/ppp/ppp.conf configuration file (the start of the default section is the best place to put it). Make sure that /etc/syslog.conf (see syslog.conf(5)) contains the lines
!ppp *.* /var/log/ppp.log
and that the file /var/log/ppp.log exists. You can now find out a lot about what is going on from the log file. Do not worry if it does not all make sense. If you need to get help from someone, it may make sense to them.
If your version of ppp(8) does not understand the set log command, you should download the latest version. It will build on FreeBSD version 2.1.5 and higher.