Why can't I connect to my bluetooth headset on a phone call on my Samsung Gear S2?

[AC Question]

I am able to successfully connect my bluetooth headset to my gear s2 and listen to music played from the watch. When I get a phone call the watch will only play the audio and use the mic on the watch. The option to use my bluetooth headset is greyed out. I have tried this with two different headsets and get the same result.