How does PrankTxt.com send an SMS from a fake number?

You enter in the "to" number and any "from" number, and the text message will send from that phone number. Not even the major SMS gateways support this, as they require all sender IDs to be pre-approved. So does anyone know how they set it up to send f开发者_运维技巧rom that particular phone number?

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In the SMS GSM protocols, the "sender id" also called OADC (Originator Address Code) is a variable. You can put any alpha numeric string with a certain max length.

Obviously this service is abusing from this functionality which is usually used for 1) branding, 2) provide a reply path for SMS a click to call number or 3) personalizing the sender for person-to-person. For instance Facebook uses it to put session ids, Skype to either put the user id or a user mobile number to provide a reply path.