Knock for Android - Tap your friend via SMS. If you still don't understand what that means then you've never used this app! Because it allows you to call via SMS! That's so unusual, but very funny. Its essence is that you write a message to my friend, and then the app makes a fake call, it called knock. Because the message itself is not always possible to hear, and if you phone rings, it's usually audible. So you will always be able to attract the attention of his interlocutor to his phone, with a simple message, and knocking about the call.