Paying on iPhone without Apple pay

My husband has a MACBook Air 13 and a iPhone. He usually pays anything on his MAC. Out of different reasons he had a payment sent as a link to his iPhone and he had difficulty to navigate to secure this payement. He received the secret security code from his bank which he validated but then the payement was refused. Mostly I suppose because there were additional security checks from the bank and he did not have time to see them before the payement was refused. The problem was to navigate, I think. I am asking this because he was trying to make the payment for a new SIM card for me with an operator that he already has a contract with.