Hi,
You will need a separate route list with two route groups, the first route group will be the H323 trunk to Avaya system and will cover your route pattern 0XXXX (for example). In route list configuration select the Avaya route group and update called party manipulation to drop predot
The second route group (within the avaya route list) will be your PSTN route group (you most likely have one of these already). While in route list configuration mode select your PSTN route group and update the called party manipulation to drop pre dot and add digits for instance 555 which will send 5550XXXX to the PSTN (you this will vary based upon your gateway SIP Tunk config)
Update your 6.0 Route pattern remove any called party manipulation, you may even have to create the avaya route list first assign to this pattern before adding the avaya H323 trunk to the route group.
To test you can update the Avaya route list, change the order of the route group to make the PSTN one first then change back. Better test would be to bring the H323 trunk down for more realistic fail over.