Ios如何验证输入的电话号码实际上是用户的电话号码?

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 have seen a couple different apps (Snapchat,