Yes, I've tried enabling the iLBC CODEC already. I opened OBi Expert configuration, disabled all CODEC's except for iLBC and still every outbound call placed through Google Voice came up using the G.711 CODEC. So I enabled syslogd and turned on sip logging and unfortunately couldn't locate the source of the issue. There wasn't enough detail in the log to tell what was wrong. Did it advertise that iLBC was a supported protocol? It appears that SIP logging doesn't provide details on the XMPP messages.
I'm left with the conclusion that even though Google Voice supports iLBC, there currently doesn't appear to be a way to configure the OBi-202 to use iLBC with Google Voice. I've opened a request with OBi Support to see if this is a supported configuration.