The latest firmware is v1.2.1 (Build 2384). ChannelTxGain and ChannelRxGain were reversed in builds prior to 2289.
If you're sure it's not Google Voice, that's a good start. If you haven't verified it recently, it might be a good idea to do so.
Have you tried a different telephone connected to the OBi PHONE Port in case it's the telephone that has the problem?
Once you've eliminated all other possibilities, then it's probably the OBi that's at fault.