What is the safest way to arrange a phone number change?

Question

Information Security

There is a service, entrance by phone number with SMS.
Let's say the user has lost his phone, SIM card, etc. (he is unable to use the number).
Rebinding is necessary, how to arrange? (passport verification is optional - someone will bind, someone will not)

Options that come to mind:
- a secret question or a code word (but very often they forget the answer)
- specify an email and restore through it

Answer 1

You can ask the user to attach a video recording with an unfolded passport and ask him to say the phrase: “I am changing the number from such and such to such and such, a secret phrase”, the secret phrase is generated at the time of requesting a change of phone number and displayed on the video attachment form . You will have to sort out such problems manually.