You may be able to do it. Bear with me on this it looks like alot, but its really not that involved.
Go into programming under terminals and sets.
Bring up a particular test set extension.
Go under user preferences
Go under button programming.
Press find
It should ask you to type a particular button.
example button 06
You will see tel# or feature
Press tel#
It will then ask for internal or external
Press external
enter the destination number
Then it will give you choices of what to use. Prime line, A particular trunk, Pool code, or routing table.
By your request, you will want to select The second choice which is typing in the trunk number you want it to use. example Trunk 003.
Alright your programmed.
When you exit out of the programming when you press that button it should dial that number you entered using line 003.
Hope that helps, and answers your question.
Kevin
Go Phils