[Asterisk-Users] Load-balancing / offload question

[Tim P]

I have 3 trunks coming into my pbx, I want one to be the main number. 
I would like to take calls coming into that trunk (trunk 1) and send
them to another trunk that is not busy (trunk 2 or 3).  Outbound is
easily handled by me assigning max channels and and outbound routing
order to each.  Incoming I want trunk 1 to always be available unless
the other two are in use.

Any way to accomplish this, has anyone done this with asterisk?