You have to select one of the OBi trunks as the Physical Interfaces -> PHONE Port -> PrimaryLine (SP1/SP2/PSTN/OBiTALK). By default, all calls will then go out that trunk. You can override this and have calls go out a specific trunk by prefxing:
**1 -> SP1
**2 -> SP2
**8 -> PSTN
**9 -> OBiTALK
If you want some automatic overrides, you have to make adjustments to DigitMaps. For example, if you want US/Canada numbers to go out SP1 and international calls to go out SP2:
Physical Interfaces -> PHONE Port -> Primary Line : SP1
Service Providers -> ITSP Profile A -> General -> DigitMap:
(1xxxxxxxxxx|<1>[2-9]xxxxxxxxx|<**2>011xx.|(Mipd)|[^*]@@.'@'@@.)
10- and 11-digit numbers will go out the default SP1 trunk, but 011+ numbers will be redirected to SP2.