I got the same issue back in last year.
They told me they will send me a letter, I said no I can provide you my other mobile phone so you can send the verification code. Then, the lady was ok with my other mobile number which is a GV number. Then, she immediately called this number to verify, lucky I was able to receive the phone call. And, she verified me and helped me to add the card to apple wallet.
2 days later, I tried to add the same card to my wife phone and I got the same issue. Called the same number again and she told me that she can’t accept my mobile number because this time the computer generated a different question. She asked that if I have a bank account with Citi, I was lucky again, I got a Citi checking account, she asked the balance, the last credit transaction and finally I got verified. Then, she helped me to add the card again for my wife phone.
So be careful when adding your card to any mobile wallet it may trigger the fraud alert and they have to verify you before you are able to do that.
Hope this helps!