Guilhem,
Ok so I'm assuming that the CCM is handling the GW features to the PBX? If so what kind of gateway, what IOS and what type of protocol is it using? Essentially you have two types of echo that can be fixed on the gateway, loud echo and long echo. I will attach a document link that explains this but will simplify what you need to do below.
Long echo can be fixed by adjusting echo-cancel coverage range on your gateway. Previous IOS versions had a default on 8ms but newer code (late 12.3T and 12.4) default that now to 64ms. On some platforms (CMM) it is even higher, 128ms. Typically, depending on IOS, you can now set that to the highest amount available due to improvements in the ECAN.
Loud echo can be fixed by adjusting you output attenuation and your input gain. Essentially you positively attenuate the signal (decrease the volume) going out to the PSTN and negatively set gain on the signal going back into the ECAN. This of course can reduce the ECHO to the level that the ECAN can account for. Of course adjusting volume can lead to a low voice volume issue for both parties. Typically you do one db each direction until you clear up the echo. On the Cisco 7960(and possibly later models) there is a tone generator so you can exactly account for and adjust the echo but this would probably not help in your scenario. Furthermore, you really need to recreate the echo for this troubleshooting to work.
With all that being said, it's probably long echo, since you aren't reporting this on the CCM phones. The additional delay from PBX->GK->CCM->GW->PSTN is probably taking you outside of your echo-cancel range. That's were you should start.
Here is some links
http://www.cisco.com/en/US/tech/tk652/tk701/technologies_white_paper09186a00800d6b68.shtml
http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080149a1f.shtml
http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801115ed.shtml
Please rate any helpful posts
Thanks
Fred
