My MRI was denied because I didn't meet medical necessity criteria. What should I do?

I've tried getting in contact with my doctor but no one picks up the phone or replies to my voicemails. It's been three straight days that I've tried calling and nothing so, should I just wait or keep calling. I have private health insurance through my job so, I can't talk to them only the doctor. I want to know what criteria did I miss and will he do the peer to peer. What should I do ?