MCK pbxgateway Administrator's Manual page 198

Troubleshooting
2. If you only experience echo occasionally on external calls, your system is
probably fine. The echo probably only occurs when calling to or
receiving calls from a party that is using a) analog lines, b) a cell phone,
or c) analog trunks on their phone system. In this case there is little you
can do. The echo canceller included with the EXTenders usually will
cancel this echo after a second or two, but occasionally the echo goes
beyond its capabilities. However, it should be noted that the party on the
other end of the call will not experience this echo.
3. If you experience a slight echo on all external calls, but it goes away after
a second or two, that is normal with analog trunk lines. If you have
analog trunks into your PBX, the echo canceller frequently takes a
second or two to detect levels of what is echo and what is voice. Then it
starts to cancel the echo. The only way to improve that situation is to
replace your analog trunks with digital trunk lines.
Issue 2.1 December 1999
6-27