Veneficus does make some good points, (for instance: mag sulfate's other uses, using a scalpel for cutting into the body, diluting meds with IV fluid...) a lot of that happens to be stuff I already knew about. I have no doubt that a Physician has a MUCH broader knowledge base than I do. I do hope, however that Vene was stating that an ETT could be placed transversely into a wound within which a large artery was severed, inflate the cuff, and tamponade the hemorrhaging and not into the lumen of a large artery, and then to inflate the cuff sealing the lumen, thusly providing a nice, smooth pathway for blood to spurt forth from the adapter of the ETT in such a way that the spurting may be more easily directed...

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Then again, I'm probably a LOT more educated than your average knuckledragging Paramedic. Clearly though, I am not a Physician, and do not claim to be.
Now to answer the point of this thread, yes, I have had physicians approach me on scene. I tell them they have 3 options. Assist me and be an additional set of eyes and hands under my direction (suggestions welcomed, but no medical control), take total control of the patient and ride to the hospital, or talk to my medical director to provide on-scene medical direction/care. I have had only ONE Physician ride in. The others usually said "thanks" and went on their way. Oh, and if I (or my crew) don't know you and you can't prove to me you're a Physician, I'm going to bounce you from my scene.
My conversation with the Base Physician (in our case) would go a LOT like medicRob's. I won't stop working on my patient while the bystander Physician consults with the Base Physician. I would expect a similar answer from my Base Doc... In my case, I've never had to contact Base for that purpose.