So, a new bit of info... My daughter just got her Model 3 this past weekend. During the delivery we found a bit of cosmetic stuff that needed detailing, so they sent the car off to get detailed while we had lunch. While they were at it, they also updated the software, though I do not know what version it got.
During her drive home she called me from the car (phone and car were linked by BT). Both her phone and mine are Google Pixel 2, and I definitely heard the echo. Her sister also reported the same (her phone is a Samsung of some sort) from an earlier call. But the new data point: During our call (about 10-11 minutes in), the echo stopped. She did nothing. I did nothing. It stopped on its own.
A few moments before I realized that it had stopped, I remember hearing a small "beep" of some kind. Could have been a notification on one of our phones, or the car, but she didn't remember hearing it. Might be related, or not; I can't say. She was also driving, though not moving much (the call was about being stuck in traffic), so I suppose she could have just switched cell towers. Can't really tell. Anyway, bottom line is that the echo can resolve itself. We just don't know triggered that resolution.