There are so many wonderful math things you can do with multiple cell phones. For example, smartphones measure angles and can triangulate distances if the gps is enabled. Or students can take, exchange and manipulate snapshots of "found math" as they go on "math treks." Think of the possibilities for explorations!
A few months ago, in several of my LinkedIn math groups, there appeared a message asking teachers to sign a petition for a law that would force an area-activated "off" switch into all cell phones. I started to investigate. The company proposing the law (and trying to sell the said switches, naturally) originally proposed it for prisons. They wanted to enforce blanket "cell phone silence" on the prison's territory. FCC said the technology is illegal and dangerous and prohibited that development. So the company turned to teachers and school administrators for support. Oh, the implications...I am happy to say the idea died by now, or at least stopped appearing in public spaces, and did not gain support from many teachers at all.
The deep issue here is whether students are considered a captive audience who won't cooperate willingly, without coercion.