Bluetooth disconnects when I make a call using Siri

My iPhone 5 and my Plantronics M1100 bluetooth headset paired with no problem. I can use the headset for most things, including Siri.

Problem is, when I use Siri over bluetooth to make a call, Siri successfully makes the call but then the bluetooth connection is immediately lost. Which means that I now am making a call, but I can't hear what anyone's saying (audio is now routed to iPhone and not even the iPhone's speaker), and the call recipient probably can't hear me, because I'm talking into the headset instead of the phone.

To be clear, bluetooth is still on, but the connection to the headset is lost.

Has anyone else figured out how to solve, or is this just an Apple issue? I had no problems with this headset with my 3GS, and no problems with this headset with my Android phone.