Nagios
What is nagios
Nagios (www.nagios.org) is an open source computer system monitoring, network monitoring and infrastructure monitoring software application. Nagios offers monitoring and alerting services for servers, switches, applications, and services. It alerts the users when things go wrong and alerts them a second time when the problem has been resolved. The voipmonitor implements GUI for nagios which you can use for any alerting / monitoring purpose.
Configuring / installing nagios
nagios debian debian 7 nagios centos 6.5 nagios from sources