Since you and I are apparently the only two who have experienced this issue, I will let you in on what I have found out.
Last night, I had a phone call with a great rep at the Kennesaw, GA Barrett Pkwy location (Jon Sharkey) who wanted to make sure I was happy with my phone. Great guy and very helpful. I said all was great expect for the speaker phone. He said to click the call to speaker phone and we tested it during the call. All was fine. He could hear me perfectly and there was no feedback. I thanked him and ended the call.
It got me wondering about how it was "corrected". I called one of the people that I had problems with before. And they still had issues with the speakerphone. Confusion set in. I called my wife, after saying hello, i switched to speakerphone and it worked great. WTH?
I have figured out that if you switch to speakerphone prior to the caller answering, then you will have these problems. If you wait for them to pick up to switch, then you can use speakerphone perfectly. Odd solution, but this seems to work great now with no problems. Try it to see if it works for you.
Hope that this helps you too!