What's wrong with the current model(s)...?
The person on the other end hears a pretty bad echo of their voice. The Z3 firmware and HD upgrade seemed to help a little but the echo is still there. My friends know right away if I'm in the car.
That's got nothing to do with the current model being unable to do it's job. That's just a software update and some common sense regarding mic placement in relation to speaker location.
If you did the update, then try either moving the mic a few inches either way, or utilizing the menu options of which speaker(s) to output the incoming audio from.
Think about what causes the far end to hear an echo...it's because the mic is picking up what's coming out of the speaker and sending it back to the far end. The software update should have some sort of echo cancellation built in, and the the rest is up to the end user to find the sweet spot for their mic. There's no one good place because each vehicle type has different acoustical characteristics............
sorry for the long winded explanation, but my background is in sound reenforcement and acoustics 