In north american systems this is enabled by default, so I assume you're elsewhere or else doing something unusual. Basically you need a secondary DT trigger shortcode, then setup your dialing shortcodes to take advantage of it. The following is a standard setup:
SC: 9
TN: .
LG: 0
F: SecondaryDialtone
Dialing 9 will now trigger secondaryDT. Now, to actually use it for anything:
SC: [9]N
TN: N
LG: <what group you want it to dial on>
F: Dial
Above will work for anything dialed after the 9. You can get very specific with the shortcodes - say you want all local calls to go over one line group and all LD to go over another. The standard north american configuration comes with most variations built in to the shortcodes already.
Note the [9] in the square brackets - this is critical. It tells the system this shortcode is paired with the secondaryDT trigger shortcode, allowing it to match against the totality of the user dialed sequence.
Peter