People cannot hear me when I talk on bluetooth headset

I am using Iphone 6 Plus (IOS 9.1) and bluetooth headset Sony MDR-AS800BT.

I have no problem on pairing and listening music, but people on the other side can barely hear my voice in a call.

I tried to maximize the volume during the call but it did not help.

I tried to pair with another bluetooth headset - Audio Technica ATH-SPORT4 and the result is the same.

I also tried testing the same with the iPhone 6s PLUS of my wife, but it did not work well too.

Any genius can help on this??

Million thanks!