1) How soon customer gets the call ? 2) It says "must enter at checkout to complete their purchase.". Where will they get the CHECKOUT page ? 3) What if customer cant hold of the computer for the next 3-4 days ? TOO much of SECURITY GENUINE ORDERS ALSO !!
They will need to enter the code at the last step on checkout after they place the payment information. Usually the page will expired in some minutes, so make sure the phone is beside you and you will listen few seconds message. Very quickly key in the code and done.
I think its better doing it. Security is the best thing. Suppose someone else got your card and shops online, nothing cannot stop him doing that. But when a notification is sent to your phone, he cannot enter the code . but the mobile will be needed to kept safe