Is either you or your wife using the phone's built in speaker phone?
Is either you or your wife using the car app that connects through the vehicle sound system?
The same questions about your son.

One thing you can do do eliminate the phones is to trying her phone in your son's car to you in your car. If that is clear than the problem is something in her car if she is using the hands free sound system.

Wait, I just remembered when I connected her phone it said I was connecting "hands free" and not the phone, per se.

Is there a way to fix that, as I recall there was only one option, it was called hands free but I dont remember seeing that before on her or my phone or car.

She is out of town, when she gets back I will try connecting it again but as I recall when the car recognizes her phone the only thing that comes up is "hands free" on the car screen, that is.

I just remembered, if I take my phone OFF the speaker in my car and talkl to her thru my phone and not thru my car, echo goes away on her end

I do not hear the echo.

Is this an actual echo of the voice being repeated twice or is it a sort of reverb sound? The latter used to be a signature sound of someone on speaker. The former is more of a problem with circuitry. We had a problem with true voice echo on our company cell phones when we were connected to a certain tower. Since I worked for Verizon Southwest and the Verizon Mobile guy depended up us to work with him - he got it fixed. If it only happening when connect to the car's sound system then the trouble is in the bluetooth or sound system if she disconnects from hands free and it happens than it is the phone. Best I can do virtually.

Generally Android phone to Android. Not sure it is only when in a vehicle.

It only happens infrequently. But talking on the phone,.. I get a 2 or 3 second delay,...., then get a loud echo of what I just said a few seconds ago.

The other party does not hear the echo and their voice does not echo as mine does.

She has a BMW, older one, 2011 and I have to put in a code to connect the bluetooth.

.... echo is resolved.