They share the same PostgreSQL database for users etc.

I've setup a single domain, sip.my-domain.com, with latency responds to the 2 servers.

So a user will always land on the server with the fastest responds in a respective area of the world.

However, it seems as if 2 users are registered on different Kamailio servers - even though they share the same database - they cannot call each other?

Have I misunderstood something?

I've enabled WITH_NAT.