Outgoing calls and extensions


I have a problem that i need to resolve.

I have now 2 zaptel channels, with a Digium card. I have 2 IP phones. OK, it?s run right, i can receive and transmit calls.

My problem is that i want to assign a exclusivelly trunk to outgoin calls to each IPphone.

For example, now a IPPhone wants to call, and if trunk 3 it?s free, it use trunk 3. If it not free , it uses trunk 4.

I want to do, that extension 200 ( for example ) always use trunk 3 to outgoing calls ( to inbound calls it?s allright ) and extension 201 always use trunk 4 to outgoing calls.

What i need to do ?

I have Asterisk@Home ( Asterisk ) with AMP , and centOS.