voipmonitor does not sniff anything

Always check if you actually see the SIP traffic. The easest way it to run (apt-get install tshark | yum install wireshark) tshark -i eth1 -R sip
if you do not see traffic, make sure that the interface is UP (ip link set up dev eth1)
if you use "-i any" and you are port-mirroring traffic, make sure you put interface to promisc mode (ifconfig eth1 promisc; ifconfig eth2 promisc;) you can put this directly in /etc/init.d/voipmonitor
Check /var/log/syslog or /var/log/messages for any problems related to voipmonitor.
Check if voipmonitor is running "ps axl |grep voipmonitor" and is using configuration file (--config-file /...)

Sniffer troubleshooting