You don't even need to add a contact. You can block any number on the iPhone by going into the Recents list in Phone and picking the 'Block this caller' option.

I've got quite a long list of blocked sales numbers, mostly Virgin Media and Three.

I use a similar feature on Android.

What I really want though is a very simple feature. I don't want my phone to accept phone calls unless the number is in my address book already. Its so straightforward and already implemented in other communication mediums, I do not understand why it is not a feature yet.

Just one vendor including this feature would force all of them to.

My parents do this the old fashioned way. They don't pickup unless they recognize the number.

In fact, my colleagues do the same, and this has actually caused problems for On call if the colleague calling at 4am isn't in the callee's circle of friends.

Android 5+ (or maybe 5.1) includes this - you can set it to priority notifications only, and then turn calls on but say to only include calls from your contacts.

Cyanogenmod, at least in the CM11 (KitKat version) offers this functionality. It also lets you know via notification what the number was.

Windows Phone goes one further: you can explicitly block any number.

Tap and hold on the number in your call history, and "Block caller" is right there!

> Tap and hold

Unfortunately, the "and hold" bit is rather hard to figure out. (And I started out knowing it could be done, but not how.)

There are some serious discoverability problems with the latest wave of pretty interfaces.

Same for Android. Add to a contact, in the contact options you can send it straight to Voicemail.

I do this on Android. The contact's name is "Asshole" :)