When you use Viber for the first time, it sends you an activaion code by SMS. I am working on the same action. After doing some research, (like "iPhone In-App SMS MFMessageComposeViewController Auto-Send" ), I have come to a conclusion but I'm not very sure it is correct. So I think some experts know the answer.
In iOS 4, we can use MFMessageComposeViewController to send SMS. However, the user will still see the SMS page and he/she needs to click 'send' button. In Viber, the user doesn't have to—he/she just gets a SMS without seeing the message page after he/she enters his/her own phone number. And all articles I read tell me iOS doesn't allow us to send SMS without user intervention.
I guess Viber has set up its own intermediate server that sends SMS. Is that correct?
Yeah, there are a number of web services that’ll send SMSs for you—Google ‘em. That’s most likely what they’re using; it’s definitely not the phone’s built-in SMS functionality.