Widex comdex problem

I have a widex comdex that for a few days when I make calls, the person I call does not hear me. But if I use it as a microphone it works. It only stops working on phone calls. Do you know what it could be?

Sometimes sediment builds up on the microphone, I use a soft brush (supplied for hearing aid cleaning) to brush the small indentation where it receives the sound to clear this.