Single most important tip for troubleshooting is inspect log files.
You'll find all the log files in
/var/log/sipxpbx
To change the logging level, log into web ui, assuming you can access the web ui, and navigate menu to System/General and you should see something like the following UI (there may be different components depending on how your system is installed and the version you are running)
Setting the log level for a component to INFO will record all the SIP messages it sends and receives in the log. The DEBUG level is very verbose, and mostly of interest to software developers; INFO is sufficient for debugging most configuration problems.