We are experiencing feedback on our T7316e phones, but only when its an external call on our POTS lines. We changed out trunk card and no luck. The feedback we are hearing only comes from our end, the caller can't hear the noise. If we use speaker phone, its fine. Unlike you problem, if we call internally, its fine, no feedback.
We tried changing to Avaya 1416 phones, but the firmware update keeps erroring out.
We've had AT&T out to check the line quality, they keep saying its fine. I'm lost on my options. I noticed today there is a quarterly update out, I'm being advised to run it to bring us from 8.0.42 to 8.0.43. I'm not holding my breath.