asterisk users - Jan 2016 - 11.21.0 : echo woes : can't installcanceller (sean darcy)

>OK. Maybe an echo canceller won't make any difference. But why does the
>remote side _always_ hear an echo if we use a local dahdi extension,
>and _never_ when we use a local SIP extension ??
The echo that the remote called hears, might be of either electrical or
acoustic origin.

If electrical, it would indicate a mismatch between the line impedance of
the Dahdi card or device and that of the phone extensions being used.  You
might be able to adjust jumpers or settings on the Dahdi card to select an
impedance which matches the phone better, and reduce the echo.  Or, use
a different type of phone, having a better-matched hybrid in it.

If acoustic, it would probably be due to bad phone handsets.  I've seen
quite a few cheap phones where sound coming out of the speaker could
travel through the handset body (like going through a pipe) and reach the
back side of the microphone and be picked up by the mike.  Better phone
handsets don't have this problem;  those that do can sometimes be improved
by stuffing the inside of the handset with some sort of damping material
such as cotton wading or Dacron pillow stuffing.

Speakerphones are, of course, notorious for generating echo.

SIP phones won't have the impedance-mismatch issue at all, and their
handsets may simply be better-designed with respect to acoustic feedback
and leakage problems.