We would be a customer if a service could do this properly - we ran into a lot of trouble with tying ours in whmcs to external api's becasue we could not get a 1-size fits all solution that would be user friendly.
The big challenge with WHMCS at least is/was that there is no way to process a new order, pause to do this fraud check (where it interacts with a user to check for a PIN code, and re-direct to re-try if say their phone didn't get the text and they want to enter a new number to verify) then once completed go back to the completed order process and process the payment/provisioning.
We spoke to WHMCS directly on it and couldn't come up with any way to make user interaction (enter pin, or retry something else) within the payment processing process of a new order.
RTGHM would be very intersted to see what you did on WHMCS, we've been considering outsourcing for a module on this but if you invented the wheel already or the start of it, maybe w can chat for more features!
my 2 cents - for us, its not about the cost - 25 cents vs 50cents per verification is perfectly fine. The cost of a charge back, or worse a spammer/bot-net person on your network causing abuse far outweighs the added cost and saves the billing dept manual time. I realize every host is different, but for us it would be invaluable to have it do everyting we wanted. We were sad to see maxmind depart this arena, it only tells me that their isn't a business case of success in the model they ran it at (which is suprising as they had built in support with whmcs and other billing systems world wide, but stil couldn't make money with their fraud verifier / phone verifier to the point they shut down the phone options even now that the main product requires payment