It sounds like you are accidentally hitting the talk button instead of the Noise Assassin button, which will hang up the call. To ensure you only tap the Noise Assassin, hold the headset on the edges, and just tap the back end. Another way to ensure you dont inadvertently hang up on the call, is to adjust the volume using your phone, as that will adjust the volume of the headset when on an active call.
Here is a visual depiction of where the two buttons are:
