Speakerphone echo and squeal

[MichaelZ]

We have a dozen people using T-26 speakerphones in separate offices and on different floors. Yet when they intercom each other, they occasionally get echo. We thought we could solve the issue by upgrading to v6.70. We were wrong. We have tried moving them to the same switch, turning on and off echo cancel and vad and jitter and adjusting volumes. They not only still can get echo. now if one side's volume is all the way down, when they turn it up, the phone squeals. This noise is not relieved by muting or adjusting the volume back down. Once it starts you can't make it stop until you end the call.

What's the best strategy for overcoming echo? squeal?