Q: Bluetooth voice call no sound

Hi all,

I got a weird issue that my iPhone 6 Bluetooth is working fine while listening to music, whatsapp voice call, whatsapp voice message, but when comes to voice calling (phone call) it becomes no sound. All the buttons to answer or finish call is working fine. I can hear beep sound while controlling the Bluetooth. I had tried up to 3 different brand and type of Bluetooth devices. All have the same issue. Also done Hard and Soft reset, restart, reset network setting.... Still the same... Please help with a proper way

THanks