Ios how can I verify that the phone number entered is actually the users phone number?

Question

I have seen a couple different apps (Snapchat, whatsapp, etc.) ask users to input their phone number. A text message is then sent to the user with a code to verify that the number is actually their number. Then they are able to see which of the users address book contacts also have the app.

I understand how all of this works except how they are sending the text messages to the user. Are they running their own SMS Gateway like Kannel, using an SMS gateway provider like twilio, or am I completely missing another option?

Seems like using a gateway provider even at 1 cent a message would add up very quickly especially when you are really only trying to protect the app from the few users that purposely put in the wrong phone number.

Answer 1

Most of the apps use sms gateway services, yes sms is costly, there is another way to verify the user number by missed call trick. http://dial2verify.com/