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If it’s a computer that hasn’t logged into that account before then it’ll be some form of multi factor authentication
It might be it gives phone number as an option to avoid inferring that there is no telephone number on the account. Similar to password reset things that say “if there’s a matching address then we’ll send it a reset link” - so that they’re not inadvertently providing a way of checking if someone has an account
Any reason that Gmail is asking for a phone number, there is no phone number on the account, to access my emails on a computer.