[asterisk-users] Asterisk Load balancing

[jk]

Hello,
we are building an asterisk cluster. Here is what we are trying.

Four Asterisk Servers
AT1, AT2, AT3, AT4.

Two service providers (SIP accounts). One for call origination (CO) and
one for call termination. (CT)
1. Say some one dials a number 18xx xxx xxxx, CO forwards the call to
our asterisk server, and the call goes to IVR, user select the option
and call go to agent.
2. User wants to call out side, Asterisk server use CT trunk and
terminate the call to the origination.

Now my concern is that only one Asterisk can register with CO and CT at
one time. Right?

I want to do the load balancing so that OC server send call to one of
the Asterisk server based on the load.

How the user registered on AT1 can make call if AT4 is registered at CT.
I have heard about Dundi and SER, but I am not if that is the right way
to go. I get the idea how a user can register at different servers. But
I am not getting how a CO knows where to send the call.
Can anyone give me some lead to get around this load balancing issue?

Thank you,
-JK