The commonly expressed notion that voice can't be taught raises a complex question about the role of a teacher in creative writing or other fields of endeavor. In my view, a teacher isn't just someone who imparts a set of rules that students can then follow to success. Certainly, that approach works for some things (and I use that approach to teach specific craft-based techniques) and for some vocations a teacher isn't really called upon to provide more. But voice is a critical component of fiction and to say "it can't be taught" is a dodge.
Let's think for a moment about the role of the teacher/mentor in myth. Joseph Campbell and other scholars in cultural anthropology, as well as philosopher-psychologists such as Carl Jung, have identified the archetype of the guide/mentor in quest stories and myths. The role of the mentor in quest stories isn't limited to teaching the hero specific technical skills needed to fulfill the quest. Though that may be involved, the mentor also helps the hero understand her true heroic nature, convincing her that she's destined or ready to fulfill the quest. In some stories, the mentor weighs in at key moments to offer more than advice.
A creative writing teacher should teach students specific technical skills, but should also serve as a guide to help the writer find within herself the components she'll need to complete her quest, and those components won't always be purely technical. Many of my lectures in workshop are intended to help writers connect with their subconscious, which is where most writers will find the answers to most of their deeper creative problems.
In one sense, the adage that you can't teach voice is correct. No one can impart voice to a writer in the same way one might a set of technical instructions. But a teacher should know how to listen, and what to listen for, and then to tell the writer where the voice sounds strong and true. This is much harder than it might appear at first glance. Rather than teaching voice, a teacher helps a writer identify her voice when she hears it, and then waits to see how the writer then develops that voice.