Outbound routing from trunk

I’ve looked through the Asterisk documentation, but can’t find a command to solve my dilemma. I have several outbound routes (different providers) and I am able to use the extension routing module for extensions to control which route. That works great. But, I have several sip trunks for remote PBX’s. So, when a call comes in SIP Trunk A, it choose the first match in the outbound routes section according to the dialed digits. Does anyone know of a configuration parameter in the trunk section (or wherever) that says SIP Trunk A can use outbound route blah?