[SR-Users] How can I make calls between the two domains?

[Yanko]

Hello:

I have two virtual machines with two kamailo domains

VM1: Domain: 172.16.16.1

Users:2000, 2001, 2002...

VM2: Domain: 172.16.16.2

Users:3000, 3001, 3002...

In each domain I can make calls. For example, user 2000 can call 2001. 
User 3000 can call 3001.

How can I make calls between the two domains?

What modules do I need to load?
How to configure the kamailio.cfg file?

Thanks,

YANKO.