why my 5s keep disconnecting from bluetooth headphones

i have a pair of hands free neckband bluetooth stereo headset. I paired it up to my iphone 5s successfully, after awhile when i receive a call, my headset vibrates normally, but when i answer the call i can't hear the other party. I can hear them thru my phone fine. I have turn my headphones off then on for it to re-connect, then i can hear thru my headset until i receive another call and the same thing happens again