I'm just following up from the above prematurely archived threads (what is with this forum doing that before an issue is resolved!). I have the same issue and must add my name to the list of those who have reported it.
I have tried this by calling my own voicemail. When I use the 510 to call myself from my VoIP line or Nokia E71, it works fine and I can hear every word I record. When I call from the iPhone, the iPhone seems to be trying to employ some sort of 'echo cancellation' and cuts out / lowers volume to zero, any words that I speak below a certain acceptable volume. In other words, I have to shout for the iPhone to 'allow' the words through.
It seems there's always some sort of issue that means allegedly compatible hardware (it says so on the Plantronics website where iPhone is listed), needs to be trashed.
Before I buy a new multipoint headset, is there any way I can make the iPhone stop lowering the volume of the 510 like this, or stop the headset doing the same if it is the cause?