You may not realize it, but echo has been a problem in the PSTN for as long as there have been telephones. You probably haven’t often experienced it, because the telecom industry has spent large sums of money designing expensive echo cancellation devices. Also, when the endpoints are physically close—e.g., when you phone your neighbor down the street—the delay is so minimal that anything you transmit will be returned back so quickly that it will be indistinguishable from the sidetone  normally occurring in your telephone.
Before we discuss measures to deal with echo, let’s first take a look at why echo occurs in the analog world.
If you hear echo, it’s not your phone that’s causing the problem; it’s the far end of the circuit. Conversely, echo heard on the far end is being generated at your end. Echo is caused by the fact that an analog local loop circuit has to transmit and receive on the same pair of wires. If this circuit is not electrically balanced, or if a low-quality telephone is connected to the end of the circuit, signals it receives can be reflected back, becoming part of the return transmission. When this reflected circuit gets back to you, you will hear the words you spoke just moments before. The human ear will perceive an echo after a delay of roughly 40 milliseconds.
In a cheap telephone, it is possible for echo to be generated in the body of the handset. This is why some cheap IP phones can cause echo even when the entire end-to-end connection ...