Having worked in healthcare, I personally know a couple hundred doctors and couldn't pronounce their names on a good day, so I call them all "Doc" and 99.9% are happy with that response. I had one doctor yell at me for calling him that and I since he was new to the facility, I kindly told him,
"I will continue to call you Doc, whether you like it or not, and no disrepect to you and what you have accomplished in your education, but in my 15 years at this hopsital, I have been punched, kicked, had urine and blood tossed at me and my experience here can't be matched for the money this place pays me. And since I specialize in what I do good luck finding a replacement. I would rather call you Doc as a sign of respect, rather than jumble up your given name numerous times. If you have a problem with that, you can always take it up with my boss and his name is proudly displayed on the plaque as you entered the building as the sole contributer to the wing you currently are standing in."
He apparrently had a talk with my boss, who kindly reminded him, that if me calling him "doc" was the biggest concern he had, he should consider himself lucky. Needless to say I never had an issue with me calling him doc, ever again.
Seriously every year we get a new batch of middle eastern doctors and there is just no way I could ever remotely pronounce them. One doctor has a last name so long (36 letters, very few vowels) that I call him doc or Dr. V
People have been messing up my name for years, I say Matt but I guess I say it too fast (even when slow) and people think I say "Mac" so after so many times trying to correct people, I just gave up, so I go by either.