How to use OTP for authentification

I have an Android App that use OTP (One time password) to authenticate the user the first time with their Phone number.

1. I want to authenticate the user automatically every time he open the app.
2. And I also want a solution for user that lose their mobile phone or user that change phone number.

How can I do this in Back-end ? Do I store the phone number ? Do I generate a token ?

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