This mod will enable you to specify the dollar amount minimum at which a customer will be contacted by MaxMind's automated telephone validation system. The customer will be instructed in the checkout_payment page that they must enter a phone number at which they will be contacted. The customer will be given a random, 4-digit code by the automated phone call. They will have to enter this random 4-digit code in the Order Confirmation page in order to complete the order.
Brian Repetti has posted an ASP script to call Telephone Verification.