Silent SMS: So quiet, that you will never hear it coming
If you have new voicemail, you get a notification on your phone. This is done by your Operator by sending an SMS that has special Data Coding parameter MWI (Message-waiting Indication). You don’t see the SMS in your inbox but your phone is able to read it and show the indicatio of new voicemail.
If you have access to SMPP supported SMS gateway, you can easily modify the same Data Coding parameter to send another kind of special SMS – “Silent” or “Invisible” SMS. The trick is to set Data Coding value to decimal 192 (hexadecimal 0xC0) and this way tell the phone to automatically discard the SMS and skip the inbox.
You still get Delivery Report that shows if the message was delivered to this phone or not, even though the owner of the phone has no idea about it. Neat.
How is “Silent SMS” used and what are the possible threats
Silent SMS is mostly used by bulk SMS providers for testing if the phone is reachable or just test out network quality but you can also make a smartphone app, that can intercept this kind of special SMS for some kind of hidden communication.
Also you have to be aware of the security issues – for example if you send constantly invisible SMS to a phone, it may be blocked from receiving other SMS or calls, as it is busy with Silent SMS.
Where to learn more technical details about the Silent SMS
For more details about Silent SMS and on how to send it, read “A Silent SMS Denial of Service (DoS) Attack” by N.J Croft and M.S Olivier. This study analyzes very well the variations and threats of Silent SMS.