I had the same problem and found these instructions posted on a Lexus IS forum:
"first hold the "info" button (I did it for about 5 secs) then switch your parking light switch from off-to parking lights-repeatedly while still holding the info button....it takes about 5-6 cycles...an poof the NAV screen enter yet another Service menu. From there I navigated to a bluetooth menu where I was able to adjust the Microphone volume higher so that people can hear me better!"
I have a 2006 GX470 and I was able to increase my microphone volume this way. I believe it is called "send voice volume" in the menu. I set it to 3 and people no longer keep saying "I can barely hear you"
Of course, this is a service menu menu so be carefull what you change.
