S.D.
I had that EXACT same issue when I had my son. The anesthesiologist that was on-call at the hospital I was delivering at, for some reason, wasn't contracted with my insurance so the bill that I received from him was astronomical. Like you, I had to send him a letter asking to reduce the bill, but my argument was that I was not given informed choice. Had I known that the possibility was there that the doc that the hospital had on-call was no contracted with my insurance, I may have been able to choose a different route or ensure that a doctor that WAS contracted be used. However, at the time of labor is NOT the time that I should be confronted with that choice as I'm in a great amount of pain and it would have caused me great distress to learn this. All of this was out of my hands. I asked that I be allowed to pay the cost of what it would have equalled to had he been contracted with my insurance. At this point, it's been a year and a half since that letter was sent and the last time I spoke with the doc's office, it was still being considered. So either it was forgiven in total or it is STILL being reviewed. I don't know if you delivered in a hospital and he was the on-call doc so you didn't really have a choice, but - at the very least - the hospital can assume some responsibility for using a doctor that isn't contracted with the same insurances that they are. That has to come up on a regular basis, I would think...
Hope this helps!