Get a referral to a speech language pathologist from your GP.
Have the SLP bill as preventative for vocal calluses (it is!).
That's it. 100% success rate for over a dozen girls on five different insurance companies among my friends. No appeals needed.
I got my insurance to pay for VFS (and FFS) (https://medium.com/@na.tashatr.oop/a-rare-victory-40408034b054). I used that same knowledge to get them to cover my daughter's breast reduction (they claimed she was too young).