http://www.freepbx.org/support/documentation/howtos/howto-resolving-audio-problems
On "sip.conf"
localnet=
nat=yes
TOOLS
Wireshark --- To Analize your computers (client) network card incoming packets
TCPDUMP --- To Analize your servers network card packets
sudo tcpdump -p -n -s 0 -vvv -w /tmp/server-capture.pcap
configuring extensions.conf
netstat -lup | grep 10