The India based websites 160by2.com and way2sms.com 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?