Make a SMS verification like Telegram in a Laravel API

I am developing an API by laravel for mobile Apps and I was looking to implement a user registration process based on the user's phone number and, for the verification of that number, I would like to implement a SMS mechanism similar to Telegram app.

As Telegram is described here, first users enter their mobile phone number and country code. then Telegram sends a code to that phone number.

Then user enters confirmation code that received. now:

After the user enters a valid activation code:

Existing users are redirected to their Chats list. New users are invited to register by entering their name and uploading a >photo. After a successful registration, users are redirected to the Chats section. In this case my Question is that how Telegram detects Existing and New users while the generated confirmation code must be >saved (in database or using other ways) while Telegram registers New users in next step (As it has said)?

Does telegram save guest users information too when entered their phone number?

Can any one to explain process of actions that in this case done?