Do you have any other BT devices, and do they stay connected? If you do, and they are not having issues, it is probably the headset.
I have two of the Plantronics Voyager Legend headsets for me and my wife. One of them kept doing exactally what you described. I took it back an exchanged it, and have not had an issue since.
The only other thing to try is deleting the headset as a device from your phone and try pairing again. Sometimes this does the trick but it sounds like a defective headset.
Piaring instructions if you need them: How to Pair Voyager Legend
I am using this headset with iPhone 5 and blackberry. It has the same problem switching back to phone in the middle of conversation. It just started doing it on both phones after 5 months of no problem. I think it is the denser inside the headset thinking I took it off from my ear and switches to phone.
Thanks for your answers.
@gwforeman: I bought another identical model, and this works fine. So it's the headset.
@Wayne: I already did that, with no success
@epekarik: That's how it went with me, too! No problems for months, then the problem started and got worse over time.
In summary it seems like a hardware problem. The last thing I can think of is resetting the headset to manufacturers setting. Did you try that, epekarik? I dunno how to do, if even possible.
I have been thru about 6 different headsets lately trying to find one that works well. this is a constant problem I have been having with mulitple models - that it just randomly drops the BT connection in the middle of a conversation. it's definitely not a headset issue as I have had it happen over and over. Does anyone have any ideas why this is happening? I have an iphone 5.