Chances are you're not doing anything wrong!
ISOtunes microphones have been designed specifically to block out steady state background noise, thereby allowing for phone calls in many loud environments, such as with vacuums, engines, machines, or saws running in the background.
However, ISOtunes microphones are not designed to eliminate impact noises, such as gun shots, so these will still come through loud and clear on calls and are likely to make conversations difficult.
A general pointer for increasing call quality: please ensure the control module (which houses the microphone) is not tucked around the back of your head, but rather under your right ear or otherwise not blocked from access to your mouth.