Too much noise.
The smartphone isn’t the problem. It is the user. (Guns don’t kill, people kill.) The ease of getting the wrong place on the internet renders it a significant risk. And that risk needs to be addressed. The level at which it needs to be approached may not be universal.
I strongly suggest that the discussion leave alone the incredible technology that the smartphone offers. It can be harnessed and directed to perform much useful, safe, and kosher service. The real matter at hand is the internet access. That, if unfettered and unfiltered, exposes the user to great risk. The otherwise frum, thoroughly observant person can have a normal moment of weakness, and venture to where he/she doesn’t belong. The yetzer horah is content to wait for this, much as a hungry lion waits for the right moment to attack.
The phone with other capabilities is not bad, dangerous, or risky. Banning it is a lame effort at trying to control others, and only serves to limit the kosher use of technology. Meanwhile, filtering technology has progressed greatly, and a smartphone with many valuable uses can have its browser blocked, and all its activity filtered. It is foolish to ban such a phone any better than to ban walking down the street because someone immodestly dressed might walk that same block.