Depends on who is hearing the echo as to how it can be cleared. In most cases it is the IP phone hearing the echo. The audio signal goesout the BRI, gets to the far end analogue exchange 4 to 2 wire conversion circuit and some of the audio is reflected back.
Therefore, the best thing to do is to drop the signal leaving the router in the first place.
Try this on the voice port -
voice-port 1/0/0
output attenuation 3
input gain -3
This will drop the output signal by 3dB so if any signal comes from the remote end it will be correspondingly lowered in level, and the input gain -3 means a negative gain, or attenuation so any return signal is reduced even further. Most users will not even percieve this drop in levels, but it is normally enough to allow the ecans to remove any remaining echo.
Refer here to some good echo theory -
http://www.cisco.com/warp/public/788/voice-qos/echo_ipphone_gateway.html
