A short message type 0 indicates that the ME must acknowledge receipt of the short message but may discard its contents.~ Page 49, GSM 03.40 Version 5.3.0: July 1996.
June 23, 2016
Short Message Type 0, also called silent SMS or Stealth SMS or Ping SMS, is used to check the availability of the receiver, and it has been around for years. The device that it is sent to “must acknowledge receipt of the short message but may discard its contents”, as is stated in the GSM technical specification. In other words, the sender knows that their message has been received, but the recipient may get no indication of this – the phone doesn’t beep or vibrate, and the message doesn’t appear in the SMS inbox.
Silent text messages can be used to secretly check whether a user’s phone is turned on or not. Seriously, there are apps that allow you to send silent texts, and you can get them for Android from the Play store. Wireless carriers and authorities, however, may use a stealth SMS to track the approximate location of a phone with the help of data received by the cellular tower.
So whether you like it or not, if you have an active cell phone in your pocket, tracking your whereabouts isn’t that hard even if you switch your GPS and data off. So the best advice for those who have paranoia, is to remove your sim from your phone and detach your phone’s battery.