This is an easy fix. I have to fix it for my wife about every 3 months.

Your phone (assuming iPhone or Android phone) has a bluetooth volume that's separate from other volumes (ringing, etc.)

The problem occurs when your phone bluetooth volume is too low so you really have to crank up the volume knob in your car to hear the other person talking. At some point you reach an inflection point and the echo starts.

So the fix:

1. Get in the car with the problem and the phone in question.
2. Start the car and start a call using bluetooth.
3. Turn up the bluetooth volume on the phone all the way. On an iPhone, this is easy - just hold down the up volume until it stops going up. Who knows on Android.
4. Now turn down the volume on the head unit in the car - in both of our 4Runners, it's about 25-30 out of 50.

That should take care of the problem.