911 goes out the LINE Port (PSTN) by default.
Assuming Google Voice is configured on SP1/ITSP Profile A and PrimaryLine = SP1 Service, the following ITSP Profile A DigitMap should send everything other than 1 + 10-digits out the Line Port:
(1xxxxxxxxxx|<**81>[2-9]xxxxxxxxx|<**8>xxxxxxxS4|<**8>011xx.|<**8>xx.|(Mipd)|[^*]@@.)
If you'd like 10-digits without a leading 1 to also use Google Voice, with a 1 automatically added, simply change the '**81' to '1' in the second rule. Otherwise, a 1 will be automatically added and the LINE Port will be used.
If you make this change to your OBi locally/directly, you must also disable Auto Provisioning at:
System Management -> Auto Provisioning -> Auto Provisioning -> Method : Disabled.
Otherwise, the OBiTALK configuration facility will overwrite your changes when you reboot. Alternatively, you can use the OBiTALK / OBi Expert configuration facility, in which case, Auto Provisioning must remain enabled.