With the system that we use, it calls the number, provides a PIN number and the user must enter that to complete the order. If the phone is never answered, there is no charge for the call. While it's more expensive if the call is going to Iraq or China etc, the $0.49 or whatever it is, is well worth it as if you have to cancel a domain registration, that's $1.00 plus whatever chargeback fees you might incur from the bank etc.
Definitely something to look into.
We notify our customers prior to the checkout process on how we do validation - we see very few order processes drop once the person starts the process.