When a call is initially presented to RouteList CallManager will reroute on ANY cause code except Out Of Bandwidth, User Busy, and Unallocated Number. The rerouting decision for those cause codes is based on the CallManager service parameters Stop Routing on Out of Bandwitch,Stop Routing on User Busy, and Stop Routing on Unallocated Number.
Once RouteList locks onto a trunk it will not attempt any reroute. The best way to discern 'locks onto a trunk' is to look for media setup. Once media setup begins RouteList is locked on the current trunk and no additional rerouting will take place.