If the ip phone is hearing echo more than likely it will be in your gateway. three things to look at adjusting is your input gain, output attenuation and your echo cancel converage.

Echo is a pain to troubleshoot i know, but if you follow the below guide, it really irons out the issues for you.

It is important to make sure that the echo canceller has enough information to distinguish between echo and voice conversation. The following parameters control the distinction:

•Input level-Signal input gain is performed before the echo canceller has detected the echo.

•Output level-Signal output attenuation is performed after the echo canceller has seen the original output signal.

•Echo canceller coverage-The amount of time the echo canceller retains a signal that has been output. This parameter must be set to a value greater than the time the echo needs to return to the gateway.

http://www.cisco.com/en/US/docs/ios/12_3/vvf_c/voice_troubleshooting/old/vts_qos.html#wp1058491