You should create the routes on outgoing routes depending the dial patterns. For example all numbers starting with 23 should go throw trunck1 one and all numbers starting with 45. So if you only have this 2 patterns and you have to sip truncks you should create a route for this pattern number
23X XXXX that should go through trunk1
and another route
45X XXXX that should go through trunk2
In reallity the dial patterns are going to be more complex, but I think this would give you an idea of how it works.