@WIY
I’ll a little exhausted from typing the above, so excuse me if I don’t give a full in-depth answer.
In short they are tricky, as are all smartphones.
For teenagers/kids please don’t buy them an Android. They’re not designed to be locked down and all solutions are basically fighting against the OS. If you absolutely must, give them an old (v2.1-2.3) device and get a real pro (e.g. TAG technician or Venishmartem) to set it up.
For adults, I would recommend an iPhone (or a Blackberry if you want blocked browser rather than just filtering) but that doesn’t help if you already have an Android, or actually need only an Android.
For Android, as with all smartphones, the method is to install an app-lock in conjunction with a new filtered browser. I won’t mention app names here because there are always new ones. Call your nearest branch of TAG.
All the methods will interfere with your phone somewhat, typically restricting the freedom to install new apps instantly whenever you want.
If you’re serious about having an effective filter you have to take things to extremes, such as rooting the Android phone or block website browsing entirely on the phone and only use it for email and suchlike, while leaving web browsing the computer.
You’ll also find, with a locked-down phone, you suddenly have double the free time for family, friends and learning!
Smartphones are a little like the internet in 1990s: filters really are not that great and will cause some trouble.
Again, if you haven’t yet bought and need a smartphone get an iPhone instead, and know it will be locked down a little.
And if your kids/teen/bochur/sem girl asks for a smartphone (or tablet, or smart media player), please refuse (especially an Android).
This information is correct at the time of posting, it may be out of date tomorrow!