Hi, I know that in my OBI200, in the Phone Interface page, there is a "ChannelTxGain" item that I can play with to increase the volume of incoming sound.
And on my phone attached to OBI200, it also has a button to increase/decrease the volume.
So presumably I can use either way to increase the sound volume. But is there any fundamental difference between these two approaches?
Yes. The OBiTALK setting is increasing the gain of the amplifier circuit in the device. Your phone is increasing the volume of its own audio output. So, they are additive.
You can experiment: set the phone to a "medium" volume. Increase the OBi's gain setting until the sound is distorted. Back down that setting until the sound quality is clear and at a comfortable level. This will allow you to adjust the volume on the phone, up or down, later, without having to mess with the OBi setting.