Very common concerns from new Asterisk, Freeswitch, opensips and
freepbx owners, How can we monitor system, what happens if service
stops responding.
Here is a small howto on monitoring any SIP service with
nagios. I am sure there are plenty of options and suggestions, but this
is one of them and has been working out very well for us for years.
http://www.didforsale.com/monitor-sip-server