Weird. It does seem that the Panasonic phone has gone bad, since as you said, and Ron pointed out, the OBi works fine with a different phone. I also have a Panasonic DECT phone plugged into my OBi, along with a Uniden 5.8GHz, and a couple of corded phones. All work fine, and all have about the same mic volume (the level that the other party hears when I speak).
Two simple possibilities: 1) some liquid or solid crud got into the Panasonic's microphone hole. These mics self-destruct if they get any cleaner (like alcohol, etc) on them (I learned this the hard way). 2) you're using a headset plugged into the Panasonic, and it's mic impedance is mismatched to the phone.
Can you plug the Panasonic into some other working phone system and try it again?
Also: there is a mistake in the current OBi firmware with regard to the labels on the Tx and Rx gain settings. They're reversed on the OBiTalk and device config webpages. It's confusing, but the correct name for the setting that controls the level of the amplifier that receives the sound from your phone's mic, and sends it out over the line, is ChannelRxGain, not ChannelTxGain (hover over the ? to see the correct definitions). OBi support acknowledged this to me and plans to fix it in a future firmware release. Meanwhile, you did (accidentally) adjust the correct value upward.
See attached screenshot for the gain values that work for me (YMMV).
