My doctor always orders one to check the baby's growth. I guess that's not medically necessary, but my insurance seems to accept that. I would definitely look around for other places to have an ultrasound.
I'm so sorry - this is so frustrating when you are expecting to have one.
Edited after reading your update from the insurance company - Phew!!! Good for you for calling the insurance company - it's all in how the doctor files the claim.