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The India based websites and allow users to signup with, and verify, a mobile number. The user can then use the website to send SMS messages to other mobile numbers with their own number as the caller id. This service is free.

They have given a lot of space on their websites for advertising (presumably to generate income). What technology are they using and how can they provide the service free of charge?

Is it possible to use VOIP to send sms with user's own mobile number as caller id? Is it correct to say they will still pay every sms they forward on behalf of their users?

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closed as off topic by Daniel Beck, Ignacio Vazquez-Abrams, random Oct 25 '11 at 5:34

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I am pretty sure these companies use ordinary SMS gateways, such as Kannel. My company is provided with this service by Beepsend and we can send SMS messages through a SOAP web service and specify any caller ID we want (alphanumericals allowed). Of course, using these services to send from a caller ID that belongs to someone else without their permission is not permitted so you'd have to have the owners permission, as in this case you have to verify it's yours.

You can read more about SMS gateways on Wikipedia.

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