I typically find cellphones to be huge distractors in the classroom.  If the teaching methodology is lecture, then I would argue that cellphones stay in the car and for emergencies students can be reached via calling in to the front desk.

However, a better approach may be to utilize cellphones in the classroom.  If there are opportunities to use them for learning and set times to have them put away and turned off, there may be better compliance.  I would argue that for most assessments structured as a series of questions to answer that cell phones need to be off during the test.  Yet a collaborative response over a blog site may get students discussing the material.

There are numerous technologies allowing cellphones (and pad devices) to be used to access and collaboratively respond to content.  Maximizing these will help the instructor and the student - as well as garner more willful compliance when it is time to put the cellphones down.

Another appraoch may be flipping the classroom.  Have students blog, review online material and discuss among themsleves outside of class and then use clas time to discuss applications and run simulations.  The instructor becomes less the lectuer and more the mentor in this scenario.