In order to help reduce or eliminate echo on calls (when using a Bluetooth headset), it is a good idea to try enabling "ECHO" (ref. 2.) cancellation on the phone. This setting can be found under the "Voice" (ref. 1.) tab within "Settings".

You may also want to confirm that the headset is at fault by trying to do test calls via the speakerphone function on the phone so you can isolate the headset.