I would start looking outside of your PBX. I know that you say you recorded the calls and they sounded fine, but problems with echo are almost always at a transition point, i.e. where the call goes from SIP to something else. I doubt it's your phones, since you use a wide variety of them and so I would look at your PRI or your PRI device.
To rule out the PBX, try using SIP trunking and see if the problem persists. If it does, then the problem is in your PBX - but I doubt it.