That number is what your bank has on file as your primary contact number. Apple doesn’t verify you, your device or and personal information. Your bank is verifying you and thus using their information on file.
>>Adding additional verification
A card issuer can decide whether a credit or debit card requires additional verification. Depending on what’s offered by the card issuer, the user may be able to choose between different options for additional verification, such as a text message, email, customer service call or a method in an approved third-party app to complete the verification. For text messages or email, the user selects from contact information the issuer has on file.
A code is sent, which must be entered into Apple Wallet, Settings or the Apple Watch
app. For customer service or verification using an app, the issuer performs their own communication process.
Card provisioning security overview - Apple Support