It's not intuitive, but "Tx gain" is the amplification of the sound being sent by the OBi (transmitted) to the earpiece of the phone attached to the OBi's telephone interface, and "Rx gain" is the amplification level of the sound being received by the OBi from the phone's microphone. Most ATAs control the audio gain using these terms.
So: experiment with the Rx gain, until your helper on the other end of the call says it's loud enough without getting distorted: not loud enough, use a higher gain number. Too loud and clipping or distorted sound, back off to a lower number.