Voice Port settings to minimize VOIP Echo

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We are trying to fine-tune our voice port to minimize echo on our VOIP
phones. Cisco techs
have been working with us, but it seems to come down to trial and
error. We have a Cisco 2851 router running CME (CallManager Express).

These are settings we have tried and the results. Any audio engineers
out there care to venture a suggestion? Thank you.

Input Gain         Output Attenuation             Result
-3......................3.....................Echo, loud and delayed
-4......................4.....................Improved, some echo
-5..................... 6.....................Worse, more echo
-4......................5.....................Improved, still some echo

Current Voice Port Echo settings:

In Gain is Set to -4 dB
Out Attenuation is Set to 5 dB
Echo Cancellation is enabled
Echo Cancellation NLP mute is disabled
Echo Cancellation NLP threshold is -21 dB
Echo Cancel Coverage is set to 64 ms
Echo Cancel worst case ERL is set to 6 dB


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