I face the same problem (Y300, FusionX, Philips (mono) Headset).
There is a workaround, which is kind of annoying though : You can pair the bluetooth headset, then make a call. As usual, the call is still using the phone. If you switch to the bluetooth settings during the call and select your headset, it should show a "phone" profile with a checkbox. Disabling the checkbox and re-enabling it will correctly route the current call to your headset.
If anyone knows a real fix, I would be very happy to know and test it