My CNM's are a practice of four and they are in the same office as a large OB practice. They bounce things off the OB's if something is out of their scope of care. With my first I never saw any OB, with my second I risked out of the hospital they practiced at and had to deliver at a larger hospital with a high risk OB.
In differs from state to state, but CNM's cannot attend homebirths nor can registered nurses as Doula's even. It jeapordizes your license.
I do know that it varies by state and even hospital though how the back up and referral to an OB works.