[Asterisk-Users] Why echo occurs

[Rich Adamson]

> 1. Is the echo (regardless of it's speed) a side effect of long
> distance communications or is it there by design for some technical
> purpose?

Echo is not there by design (with one small exception noted below).

Echo frequently is the result of imperfections in the two-wire to four-
wire hybrid.  All analog phones are two-wire devices, but most electronic
central offices and long distance facilities are essentially four-wire.
The Internet, ISDN, SIP and IAX protocols are essentially four-wire.
Each point where that two-wire to four-wire conversion takes place,
some amount of echo is likely to result. Commercial echo cancelling
hardware usually does a good job of removing the echo.

So, you might originate a call using a SIP phone, transport that call
via ISDN, but if the called end is an analog phone then a hybrid
exists at that point and could create some echo.

You'll find more detail on the wiki.

All phones (analog and digital) feed some of your transmit audio
back into the earpiece, and that is called 'sidetone'. That is sort
of an intended echo.