I found the solution for Toyota Camry 2009 at toyotanation.com. As per the page at
http://www.toyotanation.com/forum/showthread.php?p=2742471
the solution posted by Tim Harvey (tharvey) worked like a charm. Apparently the volume settings on receiving and on the microphone need to be adjusted. When I got into the bluetooth computer settings, both were set at "0". Through trial and error, I discovered that if I made the microphone (outbound) set to 3, people said I was very clear, but if I set the incoming volume higher than zero, they heard the echo of their own voice. However, other cars may need to have other settings.depending on whether the problem they are having is an echo or people not being able to hear them.
I am thrilled because I am soon to trade in my 3G for a 3GS (luckily I only bought my 3G a couple of weeks ago) and was disappointed that the car interface was so crummy.
Hope this helps everyone! For convenience, I've reposted his response, but wanted to be sure to give the link so credit goes where it is due.
Lauren Perreault
Here is his post
Had this problem and with others managed to find the solution.
Sit in car with engine off, foot off brake and press ignition button twice.
Hold down info button and turn headlight switch quickly from off to on and back several times.
After a couple of times 3-4 you will go into a service mode
Select microphone and then use arrows to go from 0 to 2
Then exit screen by foot on brake and start car and test.
I have done several some work well on 2 and unfortunately some needed 4 or even 5 to be clearly heard.
This appears only to be a problem with the 3g
It will proably be fixed in system for 3.0 in the early summer.
Good luck
Tim from Delta