When giving the credit card number to her mother she says it's a Visa card, but the first number she gives is five. All Visa cards start with four; it is MasterCard that starts with five.
When Lucy is telling her mom her credit card number she only says twelve digits. Visa cards have thirteen or sixteen.
Also when Lucy gives the Visa number she starts with a 5. Visa card numbers always start with a 4. Master Card starts with a 5.