ameenp if you just want to ask for a phone number and not use it for authentication, you could use the Masquerade extension (I think you can make the data admin-only) and make the field required. The number would then be asked after the first login automatically.
If you want to implement a new kind of authentication based on phone numbers, this should be doable with the Flarum extension system.
If there's a package ready to use for Laravel, you could use Laravel as the login provider with the Passport extension and use an external app to perform the phone login.
