If you change your Network and you find that the working Elastix / Asterisk box is now dropping calls every 15 to 25 secs. One of the most probable reason is “NAT” .
The solution which worked for me is changing the sip_custom.conf found in /etc/asterisk/ directory.
Open it with your favorite editor such as vim or nano and add the following lines.
externip=184.108.40.206/255.0.0.0 < Your External Ip with Subnet Here >
localnet=192.168.1.0/255.255.255.0 < Your Internal Ip with Subnet Here >
Hope this will help someone.