Configure Asterisk with Kamailio

I am having audio problem with phones behind another NAT (I have my Asterisk PBX inside a NAT and my phones inside another NAT).

Searching the internet, I found that this is known issue due to udp port forwarding between NATs.

I also found that we can solve this problem by using a middle man like Kamailio (OpenSER).

But I could not find how to configure asterisk with Kamailio for NAT traversal only.

Appreciate any help on this.