I'm afraid this is mostly a question of making changes and trying them out. I recommend setting volumes using a corded "plug-in" phone, as cordless dect phones have their own volume settings and may add confusion.
QuoteToday my wife took an incoming call (PSTN) and complained that her own voice echoed. Could that be down to too much Tx gain?
Echo may be caused by too high TX gain and/or mismatched impedance between the OBi and the land line. Although there is a theoretical impedance value to set the for the OBi (in the UK config file), in real life line conditions vary somewhat and trying a few other impedance settings can sometimes help. That's a last resort, changing TX and RX gains are the first step.
If you do not wish to bother your friends too much you can call 0800 500005 and listen to boring internal BT news to test your receive volume. Testing transmit is more difficult. If you have a spare phone you can plug into a socket on the PSTN side of your OBi, then you can listen to someone speaking on the phone attached to the OBi and get a rough idea of volume going to line - pick up the phone on the PSTN side of the OBi after the call is established. To do this dial the BT test number 17070 and select option 2 "quiet line test". This will remove all other noises such as dial tone while you test.
dBs as a measure of volume work in a logarithmic way, not a straight scale. Upping the level by 3dB will double the volume. Best to try 1dB at a time.