Disa-Specify which trunk to use

Hello All:

Using PBXIAF 1.3 and Freepbx 2.51

I have several friends to whom I have supplied trunks and DIDs on my box. They can call these DIDS free from their cell phone as a ‘favorite number’ and make outgoing calls via DISA.
How can I specify which trunk will be used by the DISA routine. Should I try manipulating contexts or use a custom disa routine?

Any ideas what a custom disa routine would look like?

Thanks for all ideas