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I am hearing an echo. What can I do to fix this?
Please upgrade to the latest release of Zaptel available from http://asterisk.org/downloads.  Recent versions of Zaptel include a significantly improved software echo cancellation algorithm called MG2.  We have reports of great success with this echo canceller.  Download instructions are available in the download section of http://asterisk.org.

Please verify that your rxgain and txgain are set to 0 in your zapata.conf during testing.  High rxgain and txgain settings can cause echo.

Please verify that your echocancelwhenbridged option is set to no.  Generally, it is not necessary, and in fact undesirable, to echo cancel when the circuit path is entirely TDM.  You may need to enable this only under the rare circumstance that you are unable to completely eliminate echo otherwise.

The echocancel option in the zapata.conf file only enables echo cancellation for Zap channels.  Please try an echocancel setting of 64 and an echotraining setting of 800 for testing.  Valid settings for echocancel are 32, 64, 128, and 256.  Valid settings for echotraining are 10 - 2000.  The echocancel and echotraining options may need to be tweaked in order to fully eliminate echo.  This will be a trial and error test that must be performed in order to determine the best echocancel and echotraining settings for your system.  Start with an echocancel setting of 32.  Start with an echotraining setting of 100.  Increment the echotraining setting by 100.  Do not exceed an echotraining setting of 1200.  Increment the echocancel option to the next highest setting if you still exhibit echo after reaching an echotraining setting of 1200.  Repeat this process until you find the settings that completely eliminate echo.  Asterisk must be restarted in order for these changes to take effect.

If the above suggestions do not resolve your echo problems, please uncomment the "#define ECHO_CAN_MARK2" and "#define AGGRESSIVE_SUPPRESSOR" in the zconfig.h of your Zaptel source directory.  You must comment all other echo cancellers in this file.  Then you must recompile the Zaptel source and reload the Zaptel kernel modules in order for these changes to take effect.

Digium offers two commercial alternatives for high quality, G.168-compliant echo cancellation: the software-based High Performance Echo Canceller, or HPEC, and the hardware DSP-based echo canceller available in the following digital telephony interfaces: TE412P, TE407P, TE212P, TE207P, and B410P.

The ITU G.168 specification was developed to address and standardize the performance of echo cancellers in the PSTN. This specification strictly limits the convergence time, allowed residual echo, tolerance for varying signal levels, and allowed divergence in the presence of destabilizing narrow-band energy. It also specifies the required performance of any additional non-linear processing.

HPEC is a commercially licensed, proprietary hybrid, echo cancellation solution for use with Digium and non-Digium Zaptel-compatible telephony interface cards. Its performance, in terms of its ability to eliminate echo, is superior to that of the open source routines normally included with Zaptel.

HPEC can be found at http://www.digium.com/en/products/software/hpec.php
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