In the USA:
Prescriptions are legal documents. Your doctor (with their signature) is saying they're willing to put their license on the line to say you need this.

Upon an initial decline from your insurer have the doctor re-write the prescription and include the words "Medically Necessary".
Upon the second decline, pick up the phone and call the insurer. Get whoever you can on the phone "who can help with this situation and answer a couple questions." Once that person is on the line, specifically ask for the LICENSE NUMBER of the physician that decided that it was NOT a medical necessity. DEMAND this information as you are entitled to know who on their end is Making Medical Decisions with regards to your care.

It takes a doctor to nullify a doctor. If a doctor is NOT making those decisions on their end, then someone unqualified is Making Medical Decisions about your care. I'm not sure how you feel about that, but in my state there are laws against "practicing medicine" (aka making medical decisions) without a license.

I've never had it get past the second step, but if they do provide an actual license number take that number to Google and verify that the license is valid. Be sure to ask what state they're licensed in. If asked why this information is relevant, explain to them you need to vet their doctor against your specialist. If necessary you'll need to file an ethics complaint to the medical review board and licensing state.

My experience is with BCBS, YMMV.