An echo happens when you're in between cell towers mostly. When you're bouncing off too many, or switching from one to another, you get this echo. Usually goes away, or drops the call when you finally switch.
The more towers close around you, the more likely you are to experience this. Or if you travel into different cities.
When I lived in Montana, Verizon had a tower on top of Red Lodge Mountain. So as long as I was in Red Lodge (where I lived), that was my only tower. Driving to billings is a 60mi drive, and about 45mi into the trip, I would get an echo for the last 15mi. Until I was totally in billings, relaying off their towers.