Echo is usually excessive delay and /or jitter/packet loss.

If you've got a 7940/60/70 press the '?' between the 4 buttons twice quickly during a call. It will give you some statistics to look at for voice quality. If it's a L3 problem it's just a matter of tracking down where it's at. You can track it down just using 'show interface' and pings and such to figure out where the traffic is getting dropped.

Normally echo is seen when going to the PSTN, so you want to make sure that this is happening IP phone to IP phone.

If it is, upgrade the phones to the latest firmware.

Tell them to take off their headsets as well. Recent issue with 8.4.1 phone loads with echo on some wireless headsets. Need to lower the mic volume of the headsets for that.

Good luck.

-nick