When a little red dot beside the battery symbol on my iPhone 7 appears I can’t receive or make phone calls.

When a little red dot beside the battery symbol on my iPhone 7 appears I can’t receive or make phone calls. I have to shot dawn my iPhone, then the red dot disappear and let me do calls again. But in a few minutes the red dot came back again and don’t let me do or receive phone callls.

why is that or how to repair it?