Op, you don't understand at all how the medical school teaching system works. That's ok, how could you know that stuff. I'll break it down.
Most people who teach in medical school only teach one or two lectures. If they teach a lot it's because they're famous and have tenure or are emeritus, i.e. were very good researchers and their future is secure no matter what they do. You don't get paid for teaching. It doesn't count towards your tenure or help your career except in rare exceptions. You get paid for being a clinician or researcher and bringing in revenue. Teaching does not bring in revenue. So, you'll be required to do it as a part of your position, but at most places you will not actually get paid for it. Unless you are awful or truly spectacular at it, it doesn't seem to count much. Many advise against being too good a teacher in fact, which is sad, as again this would imply that it's detracting from the real goal--bringing in revenue. A strong desire to teach does not help your career or your income, so I think most people lose that desire in academics.
When one says "professor of surgery" the reality is that person may spend some time with residents and teach a few classes a year. They may do some research and have grad students/med students under them doing grunt work. Medicine is still mostly an apprentence system. The residents follow you around and that makes you a teacher. It's not like most undergrads where the teachers teach whole courses and are paid to be teachers. Instead, in the cirriculum at most medical schools each teacher teaches one or two courses within a "block", so that they don't have to devote too much time to it and get back to making the school and themselves money.
So no, a PhD is absolutely not necessary. It all depends on what you're teaching. Basic science parts of med school are often taught by PhDs, but clinical parts of med school are usually all MDs.