The default OBi settings add "1" to any ten digit number. This is the default setting assuming your voip provider is set up on sp1:
Service Providers > ITSP Profile A > General > DigitMap:
(1xxxxxxxxxx|<1>[2-9]xxxxxxxxx|011xx.|xx.|(Mipd)|[^*#]@@.)
To achieve what you want change it to this:
Service Providers > ITSP Profile A > General > DigitMap:
(1xxxxxxxxxx|[2-9]xxxxxxxxx|011xx.|xx.|(Mipd)|[^*#]@@.)
Make changes via the OBi Expert Configuration pages. From your OBi Dashboard, click on your OBi number and follow the prompts to get there. To change a value uncheck both boxes to the right of the value and leave them unchecked. Press submit at the bottom of the page and wait a few minutes for the OBi to reboot.