Only to make sure that you’re well, you.
Giving us your phone number is a one-shot deal: you give us your digits during account setup and we send you a secure text message with a verification code. Once you verify your phone number with that code, we “hash” it – encrypt it in a way that it cannot be decrypted, even by us.
If the same number is used again, we can compare the two hashes and see that it has already been used, without knowing the actual phone number.
And that’s it – we don’t ask for it again.