Very common concerns from new Asterisk, Freeswitch, opensips and freepbx owners, How can we monitor asterisk, what happens if service stop responding. Here is a small howto on monitoring asterisk 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

Best,
-Jai