Search builder operators do not work for Preferred Communication Method
You've found a bug.  Check out Reporting a Bug, starting with step 7.

As a point of interest - variations on this bug have been reported before. For instance, CRM-6652, several years ago, which was only fixed last month with CRM-16483.

If you're looking to patch this yourself, I would definitely look to CRM-16483 as a model.