Knowing both Option-series Nortel and Avaya, I know what you're looking to do.
Depending on the release of the system (pre-4.x or 4.x), you'll be looking for Least Cost Routing or Auto Route Selection.
Pre-4.x will be LCR (4.x will be ARS), and its a nasty way to do NARS/BARS (nortel speak). Additionally, most installers of IP Office never used to read the books on proper deployment and didn't use LCR/ARS. Instead, they basically pointed "9" to the trunk group in the short codes mentioned above. Taking the easy way always burns in the long run.
The good news on that, is if they did it the SLeasy way, its a raw-handoff to the PSTN. If they were using LCR or ARS, then it would be highly unlikely they'd actually build out every NXX out there, as it is too cumbersome (how many BARS databases have you had to fill out - it sucks)

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Avaya offers exceptions-based routing for outbound calls, which is something I've always missed on Nortel, where you have to be specific (unless you just build a bunch of SPNs, but even those aren't fully exception-based the way Avaya does it). (Unable to build code, shorter code exists ---- ARGH!!!!!)
Hope you got this worked out (saw I'm a bit late on it), but a lot is going to depend on how it was initially set up. Hopefully the install vendor actually knew what they were doing and had the common sense to do the job correctly, not quickly/easily.
Hope this helps...
Kris