And as noted, any home-based filters will fail if they can connect their phones to some other wi-fi than yours. School networks are probably filtered, but of course coffee shops and other public hot spots likely are not.

At work, we used to use OpenDNS to keep porn off our network. Since we received US Govt. funds, we were required by an Act of Congress to take steps to keep porn off our network. Obviously no one can check to ensure there is no porn on every network that receives funds, there were a few "approved" methods to comply, and OpenDNS was one of them. I think it's pretty effective in general, as they do a good job of keeping their list of porn sites up to date. They also block phishing sites (web sites that mimic real-world sites in an attempt to convince you to give up the private info they are trying to get), which is (to me at least) far more valuable than blocking porn.

We of course had an enterprise account, but you can use the service for free as long as the number of queries you generate is small enough (probably most home sites would qualify). You can go to opendns.com for more info.

--Greg W.