There is an unknown number for my Apple Pay verification

Apple is not your bank. Only the issuing bank can verify your account information. The number shown is the number the bank has on file as your primary contact number. Some banks use a third party verifier, usually the Payment Network (Visa, Mastercard etc.) and that might be a number they have. Your bank would need to contact them to correct the number.

>>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